Performance Analysis of Acrylonitrile–Butadiene–Styrene Copolymer and Its Irradiated Products Under Constant and Cyclic Thermal Processes

Abstract

This study focuses on using constant and ramp cyclic processes to evaluate the performance of acrylonitrile–butadiene–styrene (ABS) copolymer with electron beam irradiation cross-linking. The main objective of this study is to compare the effects of both constant and ramp cyclic thermal processes on ABS where the results demonstrated degradation effects on ABS and its irradiated products. Under constant thermal ageing at 100 °C, the impact strength of the samples decreased drastically with increasing irradiation dosage, reaching a minimum value of 54 J/m² at 250 kGy. Tensile strength also showed a significant reduction, with values dropping from 49 MPa to 43 MPa for samples aged for 2 and 8 days, respectively, when dosages exceeded 100 kGy. This degradation is attributed to the chain scission process induced by prolonged thermal ageing and heating effects. In the ramp cyclic thermal ageing condition (Ramp-100), the impact strength followed a trend similar to the constant 100 °C thermal effect but exhibited less severe degradation. The impact strength decreased from 72 J/m² to 58 J/m² for the ramp cyclic effect compared to the greater reduction in the constant 100 °C condition (73 J/m² to 54 J/m²). This difference is likely due to the less harsh, intermittent heating ramp cyclic process compared to the continuous heating, suggesting that intermittent heating mitigated thermal degradation and chain scission mechanisms. Differential Scanning Calorimetry (DSC) analysis verified the effects of irradiation and thermal ageing on thermal properties. For unaged samples, the melting temperature remained low at 106.24 °C. With irradiation dosages of 100 and 200 kGy, the melting temperature increased to 107.76 °C and 111.43 °C, respectively, likely due to enhanced intermolecular bonding from increased cross-linking. Overall, cyclic thermal ageing caused less significant degradation of ABS products compared to constant thermal ageing. This suggests that ABS products have a longer service life in environments with ramp cyclic temperature variations compared to constant temperature conditions, which accelerate degradation.

1. Introduction

Acrylonitrile–butadiene–styrene (ABS) is a common type of polymer that comprises particulate rubber of butadiene or polybutadiene copolymer, which is dispersed in the matrix of the styrene and acrylonitrile copolymer (SAN). Over the decades, ABS has been actively involved in the electrical and electronic industry, such as the manufacture of telephones for impact resistance and aesthetic purposes (Merazzo et al., 2023) [1]. Basically, the rigidity and chemical resistance properties of a commercial ABS are attributed to the presence of SAN (approximately 70 wt%), where the toughness of ABS ultimately depends on the content of polybutadiene. However, the applications of ABS are still limited to common engineering plastics as ABS came up short of meeting the requirement of high-end engineering plastic with robust fatigue resistance. Thus, much research has been conducted to improve the properties of polymers by taking the approach of inducing a cross-linking network to the ABS matrix (Amirpour et al., 2021; Cress et al., 2021) [2,3].

Electron beam irradiation is an irradiation technique that has been commonly used as an effective alternative to induce cross-linking in the polymer matrix rather than using the traditional method of a chemical cross-linking agent. Electron beam irradiation is capable of inducing polymerisation, grafting, cross-linking, and chain scission reactions in the polymer matrix. In addition, researchers have found that irradiated polymers undergo structural changes via two important effects: cross-linking and chain scissioning (Manaila et al., 2021) [4]. In comparison with traditional chemical cross-linking agents, irradiation cross-linking is relatively faster and more versatile, thereby leading to lesser energy consumption, occupying less space for processing, and producing more uniform cross-linking in the polymer matrix (Manaila et al., 2022) [5]. Furthermore, the inherent waste-free nature of this irradiation technique makes it less polluting than conventional chemical technologies (Han et al., 2023) [6]. Recently, there has been growing usage of electron beam irradiation in industry for cross-linking of thermoplastic. By varying the rate of irradiation dosage, the rate of cross-linking and degradation of polymer can be monitored with the objective of improving the properties of the irradiated polymer (Bita et al., 2022) [7]. However, electron beam irradiation shall not be confused or mixed up with the gamma irradiation method. This might occur as both electron beam and gamma-ray irradiation are currently mostly used in industrial processes (Hassan, 2008; Xiao et al., 2024) [8,9]. Furthermore, the principles and doctrines of interaction between these two radiation techniques with matters are similar (Elnaggar, Fathy & Hassan, 2017) [10]. The only significant difference between them is the penetration power.

Electron beam irradiation on a polymer matrix generates active free radicals that are capable of bridging two long molecular chains (cross-linking) via the formation of a C-C intermolecular bond. As a result, inter-chain interaction within the polymer matrix of irradiated polymer could be enhanced effectively (Bee et al., 2021) [11]. In addition, electron beam irradiation with an inherently high dose rate can lead to a higher concentration of free radicals produced during exposure and higher recombination reactions. Due to the fact that the recombination reaction would not produce harmful product species, higher recombination would eventually lead to lesser polymer degradation and antioxidant consumption (Woo and Sanford, 2002) [12]. Electron beam irradiation was proven to cross-link more effectively to the polymer matrix than using a chemical cross-linking agent. Properties such as modulus and hardness of irradiated samples were found to be superior to chemically cured samples at all dosages (Lee et al., 2021) [13]. The mechanical, thermal, and physical properties of polymer blends can be altered by increasing the density of cross-linking in the polymer matrix via the electron beam irradiation process. Researchers believed that the gel fraction yield, tensile strength, and stress at 100% elongation were proportional to the increase in irradiation dose (Lee et al., 2021) [13]. Moreover, Liu and Tsiang (2002) [14] investigated that at the aspect of the microscopic level, structural changes such as macromolecular chain splitting, creation of low-mass fragments, production of free radicals, oxidation, and cross-linking have been found on polymers after exposure to electron beam irradiation. Furthermore, compared to pristine non-irradiated polymer samples, Liu and Tsiang (2002) [14] justified that irradiated polymers have higher glass transition temperatures as well as thermal decomposition temperatures. In addition, mechanical properties such as elongation at break of acrylonitrile–butadiene–rubber blends was found to decrease in a steady manner, while hardness and gel contents were found to increase in accordance with an increase in the irradiation dose.

The tensile modulus of irradiated polymer blends, on the other hand, is proportional to the density of cross-linking formed (Kreig et al., 2023) [15], whereas the degree of cross-linking is proportional to the amount of irradiation dosage absorbed by the polymer (Vijayabaskar, Tikku and Bhowmick, 2006) [16]. Hence, a continuous increase in irradiation dosage is accompanied by an increase in modulus. This is because, at high cross-linking density, the polymer matrix loses its mobility; thereby, more stress is needed to deform the polymer, leading to a stiffer and less ductile polymer matrix. Munusamy et al. (2009) [17] have conducted research on the effect of electron beam irradiation on the properties of the ethylene-(vinyl acetate)/natural rubber/organoclay nanocomposites (EVA/SMR L) blend. An electron beam accelerator with 2 MeV acceleration energy, 2 mA beam current, and 50 kGy per pass for a dose range of 0 to 200 kGy was used. From the tensile results obtained, the authors reported that the effect of irradiation on the tensile properties of polymer blends was not the same as compared to the trend of normal polymer blends. It was reported that the tensile strength of both pristine polymer and nanocomposite blends increased proportionally with respect to irradiation dosage (0 to 200 kGy). The improvement in tensile properties could be further supported by morphology studies on the irradiated polymer. Noriman and Ismail (2011) [18] have conducted research on the blends of styrene butadiene rubber/recycled acrylonitrile–butadiene rubber and the physical properties of irradiated acrylonitrile–butadiene rubber. The researchers have reported that the hardness value of polymer blends exhibits a similar trend to that of tensile strength. Based on the results obtained, the hardness of the polymer increased with increasing irradiation dosage and filler content due to the improved cross-linking network of the polymer matrix. In order to verify the improvement of the cross-linking network, a gel fraction test was conducted, and the results obtained showed a sharp increase in gel fraction as irradiation dosage was increased. Moreover, Landi (2003) [19] studied the impact strength of ABS polymer samples prior to and after electron beam irradiation, and it was noticed that the Izod impact strength of ABS samples was extremely high at low irradiation dosages of 0 to 25 kGy, whereas at higher irradiation dosages (100 kGy and above), the impact strength decreased rapidly with increasing irradiation dosages. The results showed that when the dosages applied were too low (0 and 25 kGy), the impact strength of polymer samples was not known because the body of the specimen did not break during impact testing, indicating very high-impact resistance. This could be attributed to the high-energy-absorbing properties of the polymer matrix, as the density of the cross-linking network formed was not sufficiently high to modify the inherent high-impact strength of ABS samples. In addition, strong interfacial adhesion by grafting the rubbery phase of polybutadiene and brittle matrix phase of the styrene and acrylonitrile polymer (SAN) provides high-impact resistance of ABS. However, as irradiation dosage kept increasing (100 kGy and above), it showed a rapid drop in Izod impact strength, indicating that the polymer matrix’s stiffness increased at the same time, becoming more fragile due to the cross-linking effect (Landi, 2003) [19].

The biggest challenge faced by the industry of plastic nowadays is the long-term stability of plastic product applications to minimise plastic waste after the end of their service life. Due to the increasing utilisation of plastic parts in applications involving UV irradiation (Ramani and Ranganathaiah, 2000) [20], severe ageing problems in products such as vehicle parts, piping, and furniture have made the long-term stability of polymers a major concern for both manufacturers and end users. By being exposed to different environmental conditions, polymers inevitably lose their properties due to intermolecular chain degradation (Santos et al., 2013) [21]. Heat, on the other hand, is one of the degrading factors, and the impacts imposed by thermal treatment are often studied by thermal ageing. Several prominent effects are initiated during thermal ageing, chain scission, cross-linking, and cross-linking breakage (Varghese, Bhagawan and Thomas, 2010) [22]. The stability of plastic is very susceptible to degradation by exposure to a high-temperature environment, which leads to oxidative degradation, stress cracking, crystallisation, and loss of volatile compounds. Degradation can take place during the drying process of polymerisation as well. Polymerisation is normally conducted in the presence of nitrogen gas so that the amount of oxygen could be suppressed to reduce the likelihood of oxidative degradation (Casale, Salvatore and Pizzigoni, 1975) [23]. For polymers such as ABS, the degradation of the polymer is initiated by hydrogen abstraction in the secondary or tertiary carbon atoms of its main chain (Guyot, 1986) [24]. For instance, in the ABS sample, hydroperoxide radicals produced via hydrogen abstraction from the α carbon atom next to trans-1,4 and 1,2 unsaturation sites in the polybutadiene phase is the main factor contributing to the thermal degradation of ABS (Bai et al., 2022) [25]. When the radicals generated combine with oxygen atoms in the air, carbonyl and hydroxyl products will be produced (Tiganis, et al., 2002) [26]. Thermal degradation of a polymer generally causes changes on its surface layer only without affecting the bulk layer. In fact, the thickness of this degraded surface layer depends on the temperature and duration of exposure to heat (Wolkowicz and Gaggar, 1981) [27]. Polymers that have been subjected to degradation due to environmental factors will have lower performance than normal polymers, and this happens particularly in rubber with an amorphous structure, which is more susceptible to degradation agents like moisture, acidic or basic conditions (Colom et al., 2024) [28]. Nguyen-Tri, Triki and Nguyen (2019) [29] reported that constant temperature ageing induced the decrement of tensile properties, such as elongation at break, due to the thermal embrittlement of the polymer matrix. In addition, morphology studies have shown that colour changes (discolouration) were limited to the surface of the aged polymer, while the bulk phase of the polymer was totally free of oxygen diffusion [30]. Furthermore, micro-indentation was noticed on the aged polymer surface, which indicates brittle features (Tiganis, et al., 2002) [26]. This concluded that ageing temperature could be used to vary the surface morphology and properties of aged polymers. If the ageing temperature falls below the polymer’s glass transition temperature (T_g), physical ageing of the amorphous polymer would take place. On the other hand, if the ageing of the polymer was conducted at a temperature higher than T_g, thermo-oxidative degradation is alleged to have taken place (Tiganis, et al., 2002) [26]. In fact, even if physical ageing and thermo-oxidative degradation in the SAN phase may contribute to the deterioration of the polymer’s mechanical properties, the effect was still insignificant compared to degradation in the PB phase (Freymond et al. 2022) [31]. Physical ageing normally occurs when the amorphous polymer is cooled rapidly from a temperature above T_g to a certain range of temperature below T_g, which provides sufficient intermolecular mobility for the rearrangement of the structure to achieve thermodynamic equilibrium. Selection of ageing conditions, especially the temperature, is vital as each of the reactants has a unique and distinct mass diffusion rate and solubility that leads to different rates of polymer degradation. Apart from the effects of elevated ageing temperature and time, thermal degradation at ambient temperature was proposed to include both surface and bulk degradation (Tiganis, et al., 2002) [26]. Thermally aged polymer samples encounter either cross-linking or chain cleavage effects. Thermally aged polymers that are subjected to chain cleavage (chain scission) have their molecular weight reduced, while cross-linking can induce embrittlement of the polymer chain. Chain scission can either take place at the chain end to form a totally new product or at any position along the polymer chain to form a lower molecular weight product (random scission). The impact strength of ABS was found to decrease upon oven ageing at temperatures of 70 °C and above (Golden, Hammant and Hazell, 1967) [32]. Wolkowicz and Gaggar (1981) [27] studied the effect of thermal ageing on the ABS terpolymer and reported that upon exposure to the heating environment, the rubbery phase of the polymer was subjected to degradation. As a result, a significant drop in impact strength was noticed in the aged polymer. Oven ageing was adopted in this experiment to heat high-impact-grade ABS under various temperatures and subsequently allow it to cool to room temperature. Upon ageing, ABS samples were taken for puncture impact testing. Based on the results obtained, when ABS was heated at a constant temperature of 190 °C, its impact strength dropped drastically at 30 min of ageing time. In fact, all ABS samples showed signs of brittle failure after 30 min of ageing time. Nonetheless, the impact strength of ABS samples was found to remain constant regardless of ageing time by heating at a lower temperature of 90 °C (Tiganis, et al., 2002) [26]. Next, by continuously heating up to 1 h, the impact strength of ABS decreased to nearly zero, which indicates complete failure. By analysing the morphology of fractured surfaces, it was observed that the thickness of the degraded surface layer increased with ageing time. A critical surface thickness in which polymer surfaces change from ductile to brittle failure was estimated at approximately 0.02 mm at 190 °C, 215 °C, and 232 °C ageing temperatures. The ultimate thickness of the degraded layer at which polymer samples experienced total failure occurred at 0.2 mm. In addition, the authors of the report investigated the molecular weight of the polymer, which drops gradually up to 30 min of ageing time but decreases rapidly after 30 min. The minor surface embrittlement of polymers, along with a very minimal molecular weight change noticed in the first 30 min, may be due to graft phase separation between the polybutadiene rubber and matrix phases. After 30 min of ageing time, chain scissioning takes place, which further deteriorates the energy-absorbing efficiency of the polymer. To date, there have been very few reports regarding the use of the thermal fatigue method to evaluate the properties of ABS, especially those subjected to electron beam irradiation at different dosages. This can help provide more understanding of ABS, especially in plastics commonly subjected to prolonged cyclic heating, to ensure they remain in an acceptable service condition.

2. Materials and Methods

2.1. Materials

The general-purpose ABS with grade TOYOLAC 500 322, produced by Toray Plastic (Malaysia) Sdn. Bhd., was used. The mixtures in the ABS resin are acrylonitrile–butadiene–styrene copolymer (95% or more), additives (5% or less), and styrene (0.05 to 0.2%). The resins used have a mass density of 1050 kg/m³ and a melt flow index of 20 g/10 min.

2.2. Formulation

Table 1. Formulation table for electron beam irradiation dosage on samples.

Sample	Electron Beam Irradiation Dosage (kGy)
S0	0
S50	50
S100 S150	100 150
S200 S250	200 250

and

Table 2. Formulation table for thermal ageing condition of electron beam irradiated samples.

Sample	Electron Beam Irradiation Dosage (kGy)	Thermal Ageing Condition
S0/CT	0	Control
S0/CS80	0	Constant-80
S0/R80	0	Ramp-80
S0/CS100	0	Constant-100
S0/R100	0	Ramp-100
S50/CT	50	Control
S50/CS80	50	Constant-80
S50/R80	50	Ramp-80
S50/CS100	50	Constant-100
S50/R100	50	Ramp-100
S100/CT	100	Control
S100/CS80	100	Constant-80
S100/R80	100	Ramp-80
S100/CS100	100	Constant-100
S100/R100	100	Ramp-100
S150/CT	150	Control
S150/CS80	150	Constant-80
S150/R80	150	Ramp-80
S150/CS100	150	Constant-100
S150/R100	150	Ramp-100
S200/CT	200	Control
S200/CS80	200	Constant-80
S200/R80	200	Ramp-80
S200/CS100	200	Constant-100
S200/R100 S250/CT S250/CS80 S250/R80 S250/CS100 S250/R100	200 250 250 250 250 250	Ramp-100 Control Constant-80 Ramp-80 Constant-100 Ramp-100

Denotation: Constant = constant thermal effect; Ramp = ramp cyclic thermal effect.

show the formulation of electron beam irradiation dosage and thermal ageing conditions on ABS samples. The electron beam is mainly applied to the ABS samples to induce cross-linking. Each of the samples was labelled SXX, with the XX representing the electron beam irradiation dosage being applied before being subjected to cyclic thermal degradation, denoted as the ramp and constant temperature.

2.3. Sample Preparation

ABS was compressed into 1 mm × 15 cm × 15 cm sheets using a Lotus Scientific L5-11009 hot presser. A dumbbell-shaped specimen was obtained by cutting the compression-moulded sheets with a Cometech QC-603A pneumatic tensile cutter according to the ASTM D1822 standard. The sample specimen prepared was an electron beam irradiated at room temperature using an accelerating voltage of 175 kV. The irradiation dosages were 50, 100, 150, and 200 kGy with 50 kGy per pass. After irradiation, the samples were then subjected to thermal ageing by using Memmert Universal Oven at four heating conditions for a duration of 2, 4, 6, and 8 days.

For the first heating ramp cyclic condition (Ramp-80), the samples were heated linearly from 30 to 80 °C for 16 h, followed by constant heating at a temperature of 80 °C for another 16 h and lastly, linear cooling from 80 to 30 °C for the last 16 h of the cycle. For the second heating condition (Constant-80), the samples were subjected to constant heating at a temperature of 80 °C, while for the third ramp cyclic heating condition (Ramp-100), samples were heated linearly from 30 to 100 °C for 16 h, followed by constant heating at a temperature of 100 °C for another 16 h and lastly, linear cooling of samples from 100 to 30 °C for the same amount of time. For the fourth heating condition (Constant-100), the samples were heated constantly at a temperature of 100 °C. Similar heating conditions were applied to the samples for a duration of 4, 6, and 8 days as well. The details of the heating process can be found in Figure 1.

2.4. Characterizations and Analyses

For the tensile test, the mechanical properties of the samples, such as tensile strength, elongation at break, and tensile modulus, were evaluated. Samples subjected to tensile tests were tested according to the ASTM D1822 standard using the Instron Universal Testing Machine 5582 Series IX tensile tester configured with Bluehill software. The test was carried out at room temperature using 2 kN of load cell and 50 mm/min of crosshead speed. The impact test was performed in accordance with the ASTM D 256 standard for the notched sample bars with different irradiation dosages and ageing conditions. The machine being used in this test was an Izod impact tester XJU-22 manufactured by Beijing United Test Co., Ltd., Beijing, China. This machine consists of a big foundation base with a clamp on the sample and ensures that the sample bar is placed in an upright vertical position during the test. The impact was exerted on the notched side of the sample bar when a pendulum-type hammer connected to the impact tester was released to break the sample bar. The energy utilised by the pendulum hammer to break the sample bar is equated with the impact strength of the sample. In order to study the melting behaviours of polymer samples, such as melting temperature and melting heat, each of the samples was subjected to Differential Scanning Calorimetry (DSC) analysis by using a Mettler Toledo DSC821^e analyser. In order to conduct this analysis, 5–8 mg of the polymer sample was heated from 30 to 200 °C at a constant heating rate of 10 °C/min. In addition, nitrogen gas was applied for purging purposes throughout the entire process at a constant volumetric flow rate of 20 mL/min.

3. Results and Discussion

3.1. Impact Test

Figure 2 shows the curve of impact strength against irradiation dosage for the ageing condition of Constant-80, where the polymers have been subjected to thermal ageing at a constant temperature of 80 °C for 2, 4, 6, and 8 days. For the unaged sample (0 days of thermal ageing), it can be observed that the impact strength increased when the irradiation dosage increased from 0 to 50 kGy and then decreased all the way down from 50 kGy to 250 kGy. In fact, the impact strength exhibited by unaged ABS samples was significantly higher than that of all the thermally aged samples. The trend of lower impact strength upon thermal ageing was consistent with the research conducted by Golden, Hammant and Hazell (1967) and Wyzgoski (1976) [32], who concluded that the impact strength of ABS was found to decrease upon oven ageing at temperatures of 70 °C and above. The improvement of impact strength upon irradiation at 50 kGy was attributed to craze formation at the cross-linked polybutadiene rubbery phase. Such a newly formed surface with an enormous craze is capable of sustaining loading by absorbing energy and toughening the material. In addition, it is estimated that a certain degree of cross-linking has occurred in the polymer matrix, enhancing the interfacial adhesion and reducing stress concentration in the notch region. This phenomenon was found to be consistent with the research that was conducted by Bee et a. (2019) [11], who concluded that a low cross-link density (<100 kGy) may improve the resistance to crack initiation and propagation of polymers. Moreover, the density of cross-linking at 50 kGy was too low to deteriorate the inherently high-energy-absorbing properties of the grafted copolymer ABS.

Meanwhile, the impact strengths of irradiated ABS samples were declining progressively as the irradiation dosage increased above 50 kGy. This could have occurred due to poor interfacial adhesion between the rubbery PB and glassy SAN phases associated with irradiation degradation at a high irradiation dosage In addition to poor interfacial adhesion, increasing cross-linking density at high irradiation dosages (>100 kGy) reduced resistance to crack propagation, thereby needing less stress intensity for crack growth). In addition, this phenomenon might be attributed to the excessively high energy of irradiation-induced intensive degradation to the polymer matrix rather than cross-linking.

Furthermore, Figure 2 also shows that at higher irradiation dosages (>50 kGy), ABS samples that underwent thermal ageing of 4 days or more had remarkably lower values of impact strength compared to samples that underwent 2 days of thermal ageing. This indicates that the crack propagation resistance and interfacial adhesion of the polymer matrix have been severely disrupted after prolonged thermal ageing. As a consequence, low crack initiation and propagation energy were expected to cause embrittlement and degradation of the material’s impact strength. It can be observed that for the first 2 days of thermal ageing, all the samples possessed a slight reduction in impact strength. This could be attributed to the reduction in interfacial adhesion between terpolymers due to the heating effect of ageing. In addition, the impact strength of ABS irradiated at high dosages (>100 kGy) was seen to be independent of the duration of ageing, as its impact strength declined at a very minimal rate throughout 8 days of ageing. This may have occurred because the polymer matrix was severely disrupted by excessively high irradiation energy. As a consequence, the linkages of the grafted copolymers PB and SAN were broken down, thereby aiding the stress concentrator notch region in failing prior to the onset of thermal ageing. In addition, the weak heating effect at 80 °C could induce minor surface embrittlement and very minimal reduction in molecular weight, therefore decreasing the impact strength of the polymer (Tiganis, et al., 2002) [26]. Meanwhile, the impact strengths of ABS samples irradiated at low dosages (<50 kGy) decreased from the beginning of day 2 to 4 and subsequently increased from day 4 to 6. It can be seen that a significant loss of impact strength occurs only after 2 days of thermal ageing, followed by a regaining of impact strength after 4 days of ageing. This could be attributed to the low heating temperature effect, which required a longer ageing period to deteriorate the interfacial adhesion of the majority of the polymer matrix. On the other hand, it is suggested that the rearrangement of the polymer chain has occurred, which caused the polymer to gain sufficient energy to withstand rapid impact loading and subsequently regain impact strength.

Figure 3 shows the graph of impact strength against irradiation dosage for condition Ramp-80, in which all the samples were heated in a cyclic manner. They were heated from 30 °C to 80 °C for 16 h, followed by constant temperature heating at 80 °C for 16 h and then cooled linearly from 80 °C to 30 °C for another 16 h. By analysing the curve, it can be seen that the trend of impact strength decreased linearly with increasing irradiation dosage, where the impact strength of Nylon 6 fell with increasing irradiation dosage. However, at low irradiation dosage (50 kGy), the ABS sample subjected to 2 days of thermal ageing showed improvement in impact strength compared to other samples (4, 6, and 8 days of thermal ageing), which possessed insignificant changes. This could be attributed to the cross-linking imposed on the polymer matrix, which strengthened the adhesion of the polymer matrix and minimised void formation. As a result, such cross-linked polymer is said to exhibit high crack initiation energy when it is subjected to impact. On the other hand, samples with thermal ageing of more than 4 days possessed no significant improvement in impact strength, which may be attributed to a reduction in interfacial adhesion effect, as the effect of prolonged thermal ageing became more significant than cross-linking of the polymer. Furthermore, the trend of impact strength against the duration of thermal ageing could be observed from the curve as well. The trend of impact strength decreased slightly after 2 days of thermal ageing and remained almost unchanged as the duration of thermal ageing was prolonged to 8 days. For the first 2 days of thermal ageing, irradiated samples of 50 and 100 kGy decreased gradually while others (>100 kGy) remained stagnant. This was attributed to the elevation of temperature, which disrupted the graft phase separation of the rubber and matrix phases, thereby leading to higher stress concentration during impact. On the other hand, the trend of impact strength for ABS irradiated at 200 and 250 kGy remained stagnant throughout the ageing process. This indicates that their polymer chains have been severely degraded by excessively high irradiation-induced chain scissioning, which eventually deteriorates the energy-absorbing efficiency of the polymer (Tiganis, et al., 2002) [26].

In addition, for ABS samples irradiated at 50 kGy, the trend of impact strength was similar to that of condition Constant-80, where the impact strength decreased after a certain duration of thermal ageing followed by increments as the thermal ageing was prolonged to 8 days. In fact, it could be noticed that the ABS samples of condition Constant-80 required a longer time (4 days) in order to recover its impact strength, while ABS of Ramp-80 spent 2 days in advance to achieve the same outcome. This phenomenon was attributed to the fact that the recrystallisation of the polymer occurred earlier for the case of condition Ramp-80, as polymer samples were cooled down after 36 h of ageing within each cycle as compared to a constant temperature for condition Constant-80.

The effect of impact strength upon irradiation at various dosages for condition Constant-100 thermal ageing can be observed in Figure 4. From the curves, it can be seen that the trends of impact strength were similar to that of condition Constant-80, where the impact strength of most of the samples decreased with increment of irradiation dosage. However, the average impact strength values for all the samples of condition Constant-100 were generally lower than those of condition Constant-80. It can be seen that the value of impact strength after condition Constant-100 fell within the range of 54–72 J/m² as compared to 57–75 J/m² of condition Constant-80. This indicates that apart from the dosage of irradiation and duration of thermal ageing, the temperature of thermal ageing is also one of the factors that could affect the impact strength properties of the material. However, in contrast to condition Constant-80, the ABS sample irradiated at 50 kGy exhibited a reduction in impact strength after 6 and 8 days of thermal ageing. This indicates that the heating effect from elevated temperatures has altered the mechanisms of the cross-linking process, as the degradation effect from thermal ageing was more dominant than cross-linking. Next, it can be observed that the impact strength of the ABS samples irradiated at 50 kGy was lower than that of the non-irradiated samples. This can be explained by the formation of crazing with microvoids and fibril-like structures along the surface of the polymer induced by chain scissioning at high temperatures (Donald and Kumar, 1982).

Furthermore, the effects of impact strength on the duration of thermal ageing for condition Constant-100 at various irradiation dosages were studied as well. From the curves, it can be noticed that the impact strength of samples irradiated at 50–150 kGy decreased progressively throughout the duration of 8 days of thermal ageing. However, ABS samples irradiated at 200 kGy experienced improvement in impact strength after 4 days of thermal ageing. This could be attributed to the recrystallisation of the polymer chains, which enhanced interfacial adhesion and reduced the stress concentration of the polymer.

Figure 5 shows the graph of impact strength against irradiation dosage for condition Ramp-100 thermal ageing. Similar to the unaged samples, when the irradiation dosage increased to 50 kGy, the impact strength of ABS samples improved. This might be attributed to cross-linking, which effectively reinforced the interfacial interaction between the grafted and polymer matrix. Next, when the irradiation dosage increased from 50 to 150 kGy, impact strength decreased rapidly. This indicates that the reduction in crack initiation and propagation resistance was initiated when chain scissioning mechanisms were more dominant within the polymer matrix. However, when polymer samples were irradiated at 200 kGy, the impact strength of the ABS samples subjected to more than 4 days of thermal ageing had an insignificant increment. While the irradiation dosage further increased to 250 kGy, all the samples experienced a fall in impact strength due to the excessively high energy of irradiation, which severely damaged the polymer chains. Figure 5 also shows the effect of impact strength on the duration of thermal ageing for condition Ramp-100 at various irradiation dosages. From the curves, the impact strength of polymer samples decreased when the thermal ageing was prolonged to 4 days. This was attributed to the breakage of the graft interface between the rubbery and matrix phases of ABS by thermally heating at high temperatures. In addition, it can be observed that the impact strength of the samples subjected to 50 kGy irradiation dosage was higher than that of non-irradiated samples. This indicates that cross-linking has promoted the reinforcement effect on the polymer interface, therefore enhancing the energy-absorbing efficiency of the polymer. When the duration of thermal ageing was prolonged to 6 days, most of the samples experienced an improvement in impact strength. This indicates samples started to undergo recrystallisation and, therefore, showed an overall increment in impact strength. As the duration of thermal ageing was further prolonged to 8 days, the impact strength fell off again as the reduction in interfacial adhesion from prolonged thermal ageing outweighed the recrystallisation effect. In a nutshell, it should be noted that the impact strength test is still a controversial method of evaluating the mechanical properties of a material as compared to other traditional methods, such as the tensile test. In fact, it relies on the notch region in the centre of the specimen to help induce crack initiation and propagation, which eventually acts as a failure point. However, it could be possible for the polymer sample to fail at any other spot rather than at the stress concentration notch region. Thus, the accuracy and reliability of the impact test in determining the mechanical properties of materials are doubtful and remain a source of scepticism for researchers.

3.2. Tensile Test

• Tensile strength

In this study, a tensile test is conducted to investigate the effect of the tensile properties of ABS with various electron beam irradiation dosages before and after thermal ageing. Figure 6 shows the graph of tensile strength against irradiation dosage for condition Constant-80 at various durations of thermal ageing. Based on the curves, it can be seen that the tensile strength for unaged (0 days of thermal ageing) samples remained higher than aged samples throughout the duration of thermal ageing. In fact, the tensile strength of unaged samples increased drastically from the beginning (0 kGy) and reached a maximum of around 100 kGy but decreased as irradiation dosage increased further from 100 to 250 kGy. The trend displayed by unaged samples was consistent with the findings of Vijayabaskar, Tikku and Bhowmick (2006), indicating that the tensile strengths of materials increased with an increment in irradiation dosage, followed by a reduction at a higher dosage. This increment of tensile strength from 0 to 100 kGy could be attributed to cross-linking of the polymer, whereas the fall of tensile strength at higher dosages (>100 kGy) was attributed to irradiation-induced chain scissioning. As a matter of fact, cross-linking induced by a certain range of irradiation energy would increase the molecular weight and intermolecular bonding of the polymer matrix. Hence, stable polymer chains with a higher degree of entanglement formed to restrict the sliding of macromolecular chains during stretching, thereby leading to a relatively stronger tensile strength. Chain scissioning, on the other hand, reduced the molecular weight of the polymer. The resulting polymer has a shorter chain and lesser entanglement, thereby allowing easy slippage during stretching and, therefore, lower tensile strength.

For aged samples subjected to 0 to 100 kGy irradiation dosages, the tensile strength of samples increased due to the restriction of intermolecular chain sliding from cross-linking. However, when the irradiation dosage increased up to 150 kGy, most of the aged samples experienced a loss of tensile strength. This was attributed to the ease of intermolecular sliding during stretching by the chain scissioning effect. Despite chain scissioning occurring at 150 kGy, a further increase in irradiation dosage from 150 to 250 kGy significantly improved the tensile strength of aged ABS samples from a range of 44–47 MPa to 48–52 MPa. This could be attributed to cross-linking induced by irradiation becoming more dominant than chain scissioning; therefore, more stress is needed to break the larger and higher entanglement polymer chains. In addition, the tensile strength of the non-irradiated samples was significantly lower than that of the irradiated samples throughout the duration of thermal ageing. This indicates that cross-linking upon electron beam irradiation improved the tensile strength of samples despite significant loss of tensile strength upon thermal ageing. The trend of tensile strength of all samples decreased upon thermal ageing for 2 days, followed by a slight increase from 2 to 4 days. The loss of tensile strength for the first 2 days of thermal ageing could be attributed to the heating effect of thermal ageing, which decomposed polymer chains that have relatively weaker intermolecular bonding. On the other hand, the slight improvement in tensile strength at 4 days of thermal ageing indicates that a small scale of recrystallisation occurred where molecules gained sufficient energy to rearrange themselves into a uniform and stable polymer structure, which resisted easy slippage during stretching. As the duration of thermal ageing was prolonged to 4 days and above, the tensile strength of the polymer samples decreased again. This could be explained by the breakage of polymer chains, as thermal degradation appeared to be more significant than the recrystallisation of chain molecules.

Furthermore, it could be noticed that the tensile strength of samples with high dosages (>200 kGy) was significantly higher than that of samples with low dosages (<200 kGy) when the duration of thermal ageing was prolonged for more than 2 days. This was attributed to the higher degree of cross-linking density in the polymer matrix of high-dosage samples induced by thermo-oxidative degradation (Tiganis, et al., 2002) [26]. On the other hand, the thermal decomposition of low-dosage samples to shorter and weaker chains via chain scissioning became more significant after prolonged thermal ageing.

Figure 7 shows the graph of tensile strength against irradiation dosages for condition Ramp-80. Similar to unaged samples, it can be seen that the trends of tensile strength for all the aged samples increased gradually with irradiation dosages from 0 to 100 kGy. This can be explained by the cross-linking effect, which strengthens the entanglement and intermolecular bonding of the polymer chain. Next, samples subjected to 2 and 8 days of thermal ageing experienced a slight loss of tensile strength when irradiation dosage increased up to 150 kGy despite the improvement in tensile strength shown by samples aged for 4 and 6 days. This could be attributed to the degradation of weak polymer chains when they were first heated at elevated temperature, whereas when thermal ageing was prolonged to 8 days, chain scissioning of the polymer matrix became more dominant than cross-linking. When the irradiation dosage was higher than 150 kGy, tensile strength started to increase again before falling off slightly at 250 kGy. This phenomenon was similar to the case of condition Constant-80, as a certain degree of cross-linking had occurred, which led to an improvement in tensile strength. However, compared to condition Constant-80, the tensile strength values of polymers were significantly lower. The tensile strength value after condition Ramp-80 fell within the range of 39–45 MPa, as opposed to 42–52 MPa of condition Constant-80. This indicates that apart from the effect of irradiation dosage and duration of thermal ageing, the temperature of thermal ageing was one of the crucial factors in manipulating the tensile strength behaviour of the material as well.

In addition, Figure 7 also shows the trend of tensile strength as a function of the duration of thermal ageing for condition Ramp-80. For the first 2 days of thermal ageing, the tensile strengths for all the samples fell drastically. This could be attributed to the decomposition of weaker polymer chains by the heating effect of thermal ageing. It could be noticed that the tensile strength of irradiated samples was significantly higher than that of non-irradiated samples. This indicates that despite the cross-linking effect initiated upon electron beam irradiation of polymers, the degradation effect from thermal ageing was more dominant than cross-linking. Next, similar to condition Constant-80, the tensile strength of the majority of samples improved after being thermally aged for 4 days. This phenomenon was attributed to the recrystallisation effect which strengthened the intermolecular bonding of polymers through the formation of a stable crystalline structure. Thus, more stress is needed to stretch the polymer chains with stronger bonding and structure. Apart from that, when thermal ageing was prolonged to 6 days and above, the tensile strengths of most of the aged samples remained constant or showed insignificant changes as compared to ageing at 2 and 4 days. This can be explained by no further recrystallisation occurring within the polymer matrix after a long period of ageing.

The effect of various irradiation dosages on the tensile strength of polymers subjected to thermal ageing for 2 to 8 days at a constant 100 °C is explained in Figure 8. From the curves, it can be seen that the trend of the tensile strength increased with irradiation dosage up to 100 kGy. The increment in tensile strength upon irradiation up to 100 kGy was attributed to the higher molecular weight and stronger entanglement of the polymer matrix upon cross-linking by the electron beam. When irradiation dosages further increased up to 250 kGy, tensile strength fell off significantly. This could mean that chain scissioning mechanisms have occurred and were more dominant than cross-linking at high irradiation dosages (>100 kGy), thereby reducing the size and entanglement of the polymer matrix. It can be seen that at a lower dosage (50 kGy), the tensile strength of the sample subjected to 8 days of thermal ageing fell off instead of improving, as shown by other samples. This indicates that the long duration of thermal ageing decomposed the intermolecular bonding of polymer chains, which led to lower tensile energy during stretching. Nevertheless, compared to condition Constant-80, the degree of tensile strength reduction was significantly higher. This phenomenon could be attributed to the higher heating effect of condition Constant-100, which subsequently altered the chain scissioning and cross-linking mechanisms of the polymers. Thus, it could be presumed that the temperature of thermal ageing is one of the factors that could alter the tensile properties of material despite irradiation dosage and duration of thermal ageing.

Furthermore, it could be observed that for samples subjected to 2 and 8 days of thermal ageing, the trend of tensile strength curves was consistent with unaged samples. In addition, when thermal ageing was prolonged to 6 days, tensile strength increased gradually. This can be explained by the fact that the recrystallisation of the polymer structure had started, which allowed them to restructure and gain sufficient energy to prevent easy slippage upon tensile stretching. When thermal ageing was prolonged by more than 6 days, all the irradiated samples had their tensile strength reduced significantly. This could mean that cleavage of grafted polymer chains has occurred after an overly long period of ageing duration. This phenomenon was consistent with the research by Tiganis et al. (2002) [26], who concluded that the tensile stress of ABS samples significantly deteriorates at a higher temperature and ageing duration.

For condition Ramp-100, the graph of tensile strength against irradiation dosage is shown in Figure 9. Based on the curves, the tensile strengths of samples increased gradually up to 150 kGy, followed by a reduction at higher dosages. It can be observed that the tensile strength of samples thermally aged for 2 days increased at a significantly high extent compared to that of other samples with longer ageing periods. This phenomenon could be attributed to the fact that the intermolecular bonding and entanglement of the polymer matrix were still high and intact after 2 days of ageing. In contrast, as the duration of thermal ageing was prolonged, the polymer matrix lost its intact bonding and chain entanglement, thereby allowing easy stress stretching. When irradiation dosage increased beyond 150 kGy, polymers lost their tensile strength properties due to the fact that, under such ageing conditions, chain scissioning was more dominant than cross-linking at higher dosages (>150 kGy). Nevertheless, it could be noticed that samples thermally aged for 8 days possessed different trends compared to other samples. This could be attributed to the uneven heating and cooling effects of ramp condition thermal ageing.

Figure 9 also explains the variation in tensile strength under various durations of thermal ageing under condition Ramp-100. From the curves, it could be observed that the tensile strength for all the samples was pretty consistent and seemed to be independent of the dosages of electron beam irradiation. Next, a trend of decreasing tensile strength was noticed when the duration of thermal ageing was prolonged from 0 to 4 days for every single polymer sample. This indicates that the degradation of polymer chains due to high-temperature ageing was so dominant that the effect of cross-linking on polymers was nullified. As thermal ageing was prolonged to 4 days and beyond, polymer samples regained tensile strength and subsequently increased with time due to the recrystallisation effect, which reinforced a stable crystalline structure. After 8 days of thermal ageing, the tensile strength of the samples eventually varied or became dependent on their respective irradiation dosage compared to short-duration ageing. It is observed that samples irradiated at higher dosages experienced stronger tensile strength, which was consistent with the trends exhibited by irradiated polymers. This phenomenon explained that there was no further intensive thermal decomposition of the polymer matrix; therefore, cross-linking became more significant than the thermal degradation effect.

• Elongation at Break

The effects of thermal ageing and various irradiation dosages on elongation at break of the polymer samples are shown in Figure 10. Based on the curves, it can be noticed that the values of elongation at break of the samples varied progressively with increasing irradiation dosages. For the unaged sample, elongation at break increased drastically and reached a maximum value at 50 kGy. This phenomenon can be explained by a certain degree of cross-linking networking in the polymer matrix, which enhances the flexibility of polymer chains during stretching. When the irradiation dosage increased up to 100 kGy, elongation at break of the samples decreased. This could be attributed to the random scissioning of the polymer matrix, which led to easy breakage with lower elongation. Next, at an irradiation dosage of 150 kGy, elongation at break of the samples showed significant improvement due to a higher cross-linking network in the polymer matrix. At higher dosages (>150 kGy), elongation at break of the samples remained constant with a slight decrement in between. This phenomenon could be attributed to the chain scissioning mechanism induced by high irradiation energy, therefore leading to chain cleavage. In addition, it could be noticed that elongation at break of all the aged samples remained at a lower value than the unaged samples for the entire range of dosages. This was attributed to the restriction of chain mobility upon irradiation on the polymer matrix. Moreover, at low dosages (<150 kGy), the elongation at break of samples aged for 2 days remained at relatively low values as the effect of thermal degradation upon ageing was more significant than cross-linking of irradiation dosage. When dosage increased for more than 150 kGy, elongation at break improved gradually as cross-linking density was more dominant.

Other than a variation in irradiation dosage, Figure 10 also describes the effect of duration of thermal ageing on the elongation at break of samples under condition Constant-80. From the curves, it can be seen that upon thermal ageing for 2 days, elongation at break for samples decreased rapidly due to the thermal embrittlement induced in the glassy matrix of the SAN phase (Golden, Hammant and Hazell, 1967) [32]. Furthermore, as the duration of thermal ageing was prolonged to 4 days, samples showed a slight improvement in elongation at break. This indicates that under proper conditions and duration of thermal ageing, the orientation of chain structures would be modified in a certain way to improve chain mobility. As thermal ageing was prolonged to 4 days and above, elongation at break fell off significantly. This can be explained by the alteration of the chain scissioning mechanism after a long period of thermal ageing, which leads to easy fracture of samples at low elongation.

Figure 11 shows the trend of elongation at break versus irradiation dosage for thermal ageing of condition Ramp-80. It can be observed that the elongation at break decreased upon irradiation at 50 kGy. This was attributed to the degradation of the polybutadiene phase, which eventually reduced the chain mobility of polymers, thereby causing the polymer to elongate less at fracture. As irradiation dosages increased up to 150 kGy, values of elongation at break of the samples were noticed to increase gradually. This indicates the formation of the cross-linking network within the polymer matrix induced by higher irradiation energy. When the dosage of electron beam irradiation further increased to 200 kGy, samples subjected to more than 2 days of thermal ageing had their elongation at break decreased. This was attributed to an altered chain scissioning mechanism at high irradiation dosage, which eventually caused the cleavage of polymer chains. Above 200 kGy, elongation at break improved slightly again as interfacial interaction within the polymer matrix was promoted, easing the chain movement while mitigating chain restriction. In addition, it could be noticed that for ramp temperature ageing, elongation at break in samples was independent of the duration of thermal ageing at low dosages (<100 kGy), which opposed the trend showed by condition Constant-80 ageing. This could be explained by the periodical heating and cooling effect of the ramp condition, thereby requiring polymer samples to take relatively more time to vary the chain mobility compared to constant heating.

Furthermore, Figure 11 also shows elongation at break against the duration of thermal ageing for condition Ramp-80. Based on the curves, it was noticed that the trend of elongation at break was pretty consistent for all the samples. As thermal ageing was prolonged to 4 days, elongation at break decreased gradually due to embrittlement of the glassy phase matrix, thereby making polymer chains more prone to breaking at fracture. When thermal ageing continued up to 6 days, elongation at break started to bounce up slightly as the polymer matrix gained sufficient energy and strengthened upon cross-linking. With a stronger polymer structure, a greater amount of stress is needed to elongate the samples, which leads to larger strain and eventually fracture. After 6 days of thermal ageing, there were no significant changes in the elongation at break for most of the samples due to the fact that there were no further weak polymer chains for chain scissioning to take place.

The effect of elongation at break upon various irradiation dosages for thermal ageing at a constant 100 °C is shown in Figure 11. Basically, the polymer samples subjected to 4 and 8 days of thermal ageing exhibited a similar trend of elongation at break to that of the unaged samples when the irradiation dosage increased from 0 to 250 kGy. However, thermally aged samples of 2 and 6 days, on the other hand, possessed significant reduction in elongation at break when they were first irradiated at 50 kGy. This could be attributed to the random scissioning mechanism, which allowed easy cleavage of chains during stretching, thereby lowering the elongation prior to fracture. When dosage increased to 100 kGy, both of the samples experienced higher elongation at break, followed by rather stable values of elongation at higher dosages (>100 kGy). These phenomena indicate the modification of polymer structures and alteration of chain scission mechanism. Moreover, it can be seen on the curve that the sample subjected to 2 days of thermal ageing had the highest elongation before irradiation, followed by gradual loss and eventually remained at a range of values higher than other aged samples. This phenomenon can be explained by the alteration of the chain scissioning process at dosages higher than 50 kGy, which promoted the flexibility of chain movements.

In addition, by analysing Figure 12, the trend of elongation at break at different durations of thermal ageing for condition Constant-100 could be evaluated. According to the curves, the elongation at break of the non-irradiated sample increased sharply from 15 to 27% after being thermally aged for 2 days, whereas the irradiated samples were found to have slightly lower elongation values. This might be attributed to the insignificant effect of the chain scissioning process at the early stage of thermal ageing, thereby allowing polymer chains to have intact mobility and structures. When the duration of thermal ageing was prolonged from 2 to 6 days, loss of elongation was experienced by all the thermally aged samples. This suggests that the degree of embrittlement of the glassy matrix phase of ABS increased with a prolonged duration of thermal ageing. With a brittle and cracked scattered structure, the resulting polymers could be easily broken when subjected to stress. For the sample irradiated at 50 kGy, elongation at break reduced severely from 27 to 12% for 6 days before bouncing back to 17% after 8 days. The rapid fall of elongation at break could be attributed to the intensive scission mechanisms, eventually leading to progressive cleavage of chains with time. Nevertheless, the increment in elongation at break after 8 days indicates no further chain scissioning, as the molecular structures started to rearrange themselves into a more ordered state, thereby mitigating the restriction of polymer chain mobility.

Figure 13 shows the trend of curves for elongation at break against various irradiation dosages for different durations of thermal ageing at condition Ramp-100. It can be seen that the values of elongation at break for most of the samples possessed no significant changes for the whole range of irradiation dosages. For samples irradiated at 50 kGy, the elongation at break was relatively low compared to the extensively high elongation at break showed by the non-irradiated samples and the samples thermally aged under condition Constant-100. This can be explained by the limited chain movement of polymers under on-and-off heating environments during ramp condition thermal ageing. When irradiation dosages increased from 50 to 150 kGy, all the samples possessed remarkably constant values of elongation at break as compared to significant variation under condition Constant-100 thermal ageing. This indicates that elongation at break of the samples was independent of either variation in irradiation dosages or duration of thermal ageing. At higher dosages (>150 kGy), the variation in elongation at break was more profound despite the insignificant effect of thermal ageing duration.

Furthermore, Figure 13 also shows the elongation at break as a function of thermal ageing duration for condition Ramp-100 thermal ageing. Based on the curves of the graph, elongation at break of the polymers decreased drastically after they were subjected to the first thermal ageing cycle (2 days). This indicates chain scissioning induced by thermal embrittlement of samples at elevated temperatures. As thermal ageing was prolonged to three cycles (6 days), elongation at break of the samples decreased in a progressive manner, followed by stagnant and consistent values during the last cycle of thermal ageing. This could mean that as thermal ageing was prolonged, the thermal decomposition of molecular chains increased as well, and therefore, shorter elongation and easy breakage would occur prior to fracture, while during the last cycle of thermal ageing, no further changes were observed, as no weak polymer chains remained to be decomposed. It can be observed that for the non-irradiated sample, the elongation at break value was relatively lower than that of the irradiated samples during the early cycles of thermal ageing. This could be attributed to a more dominant random scissioning process, which impedes the elongation of polymers prior to fracture. In other words, chain scission mechanisms were less significant for irradiated polymers at a short duration of thermal ageing. However, as thermal ageing was prolonged to higher cycles (4 days and above), elongation at break of the non-irradiated samples bounced up to a higher value than the remaining irradiated samples. This concluded that the degree of chain scissioning on irradiated polymers was more profound at a prolonged duration of thermal ageing. For samples irradiated for 50 kGy, it could be observed that the elongation at break decreased at a range of 27–14%, compared to 27–11% under condition Constant-100. The extent of elongation at break loss was slightly lower due to the periodical heating and cooling of samples, which resulted in polymers fracturing at shorter elongation.

• Tensile Modulus

Figure 14 shows the curves of tensile modulus versus various irradiation dosages of thermally aged samples under condition Constant-80. Based on the curves, it could be observed that the tensile modulus of the unaged sample increased drastically at low dosages (<100 kGy). This could be attributed to the increment of cross-linking density at higher irradiation dosages. Thus, polymer chains lost their mobility as they were entangled into larger molecule sizes, thereby requiring more stress to deform the polymer, indicating a stiffer and less ductile polymer matrix. As irradiation dosages increased from 100 to 200 kGy, there were no significant changes in tensile modulus as they were maintained at values of about 175 to 177 MPa. When the dosage further increased to 250 kGy, the tensile modulus decreased as chain scissioning induced by high irradiation energy was more dominant. Hence, shorter and smaller polymer structures formed would have better flexibility and lesser restriction to slide past the chains of each other; thus, lower stress is required to break the samples.

Similar to unaged samples, all the aged samples were found to have their tensile modulus increased with irradiation dosages of up to 100 kGy. The tensile modulus of the sample subjected to 4 days of thermal ageing increased to the greatest extent, from 145 to 186 MPa, after irradiated at 100 kGy, whereas, for the sample subjected to 8 days of thermal ageing, the increment in tensile modulus was the least, from 145 to 153 MPa. This phenomenon indicates the chain mobility of the polymer subjected to 4 days of thermal ageing was restricted by a large density of cross-linking, whereas the sample that was thermally aged for 8 days decomposed into shorter structures with better chain mobility. At high dosages (>100 kGy), the tensile modulus of the aged samples decreased gradually as a result of better chain flexibility induced by a high cross-linking density. By comparing with the non-irradiated polymer samples, it can be seen that the tensile modulus of the irradiated samples was relatively higher than that of the non-irradiated samples. This could be attributed to the overall improvement in the modulus through cross-linking from thermo-oxidative degradation (Tiganis, et al., 2002) [26].

Figure 15 shows the curves of the tensile modulus against various irradiation dosages for condition Ramp-80 thermal ageing. For the non-irradiated samples, the tensile modulus of the samples subjected to 6 days of thermal ageing was the highest at a value of 139 MPa, followed by samples with 4 and 8 days of thermal ageing at modulus values of 129–137 MPa, and lastly, the sample subjected to 2 days of thermal ageing. The increment in the tensile modulus upon thermally ageing from 2 to 6 days could be explained by the rearrangement of molecules from an unordered melt structure to a more uniform and stable structure. Furthermore, the initial loss of the tensile modulus from 159 to 126 MPa after 2 days of thermal ageing could be attributed to the decomposition of long chains into shorter and more flexible structures when the polymer first started to heat. When the irradiation dosage increased up to 200 kGy, the tensile modulus of samples was pretty consistent with unaged samples where the modulus value increased in a progressive manner. However, it could be noticed that the polymer thermally aged for 2 days had its tensile modulus improved rapidly at 100 kGy and sooner possessed the highest modulus at 200 kGy. Thus, a larger polymer with a rigid three-dimensional cross-linked network was formed to restrict the chain mobility and prevent easy sliding of intermolecular chains during stretching. At higher dosages (>200 kGy), the tensile modulus of all the polymer samples decreased due to the chain cleavage caused by the dominant chain scissioning mechanism induced by high-energy radiation.

Figure 16 shows the tensile modulus versus irradiation dosage of samples subjected to condition Constant-100 thermal ageing. Based on the curves, it can be seen that the tensile modulus exhibited by the non-irradiated sample was consistent with the modulus trend of Ramp-80 thermal ageing, where the samples that were thermally aged at 4 and 6 days possessed a higher tensile modulus than other samples subjected to 2 and 8 days of ageing. Furthermore, when irradiation increased to 50 kGy, all the samples possessed similar tensile modulus values despite being subjected to different durations of thermal ageing. When irradiation dosages increased from 50 to 200 kGy, the tensile modulus of samples increased gradually before falling off at 250 kGy again. It was also notable that samples subjected to 6 days of thermal ageing showed a significant improvement in tensile modulus from 100 kGy and above. However, compared to condition Constant-80, the increment in tensile modulus for condition Constant-100 was significantly lower. This was attributed to the strict heating environment of condition Constant-100, which impeded the intermolecular bonding of the polymer matrix, eventually allowing easy sliding between molecules (Bee et al., 2019) [33]. At a high irradiation dosage of 250 kGy, the tensile modulus of the samples was consistent, irrespective of the duration of thermal ageing. This phenomenon further explains the fact that chain scissioning had taken place in most of the polymer matrix, which led to lower stress needed to deform the sample.

Figure 17 shows the curves of the tensile modulus for thermal ageing of condition Ramp-100. Based on the curves, the trend of the tensile modulus for the non-irradiated sample decreased after being thermally aged for 4 days. This may be attributed to the breakage of the chain induced by altered chain scissioning mechanisms. Next, the tensile modulus increased after being thermally aged for 6 days, followed by a slight reduction after 8 days. The improvement in tensile modulus noticed after 6 days of thermal ageing could be attributed to the rearrangement of molecules into uniform structures, thereby reducing the chain mobility during stretching. When the irradiation dosage increased to 100 kGy, the trend of the tensile modulus for most of the samples increased, then flattened out at 150 kGy. The improvement in the tensile modulus properties of the samples was consistent with the research conducted by Tiganis et al. (2002) [26], where the tensile modulus of polymers increased as a result of cross-linking in the rubbery polybutadiene phase. Therefore, the phase transformation from ductile to brittle was noticed prior to failure. At high dosages (>200 kGy), the tensile modulus decreased significantly. This could mean that the breakage of polymer chains became more significant than cross-linking at high dosages. Thus, shorter polymer chains with weaker intermolecular bonding would be formed.

3.3. Differential Scanning Calorimetry

Differential Scanning Calorimetry is one of the most widely used thermal analysis methods to study the thermal stability and melting behaviour of polymers subjected to high-temperature heating. In this study, the thermal properties of polymer samples, such as melting temperature (T_m), were obtained from the peak of the DSC thermograms curve, whereas the melting heat of polymers was determined by the area under the curve of thermograms.

Table 3. Melting temperature of samples subjected to condition Constant-100 thermal ageing at various irradiation dosages.

Irradiation Dosage (kGy)	Melting Temperature (°C)
	2 Days Thermal Ageing	4 Days Thermal Ageing	6 Days Thermal Ageing	8 Days Thermal Ageing
0	108.1	108.3	109.25	109.88
100	108.56	111.14	111.1	110.76
200	111.41	111.14	111.99	111.25

shows the curve of T_m of irradiated ABS samples of 0, 100, and 200 kGy after thermal ageing for 2 to 8 days at a constant temperature of 100 °C. It can be observed that the results of T_m against various irradiation dosages were consistent with the research conducted by Liu and Tsiang (2002) [14], in which the melting point of the polymers increased marginally with higher irradiation dosages. This phenomenon could be attributed to the formation of stronger intermolecular bonding within the polymer matrix as a result of the cross-linking effect. Thus, the resulting polymers, with their structure modified, became more resistant to melting. As the duration of thermal ageing was prolonged from 2 to 6 days, the polymer samples showed a slight increase in T_m of up to 3 °C. This can be explained by the fact that as the duration of thermal ageing increased, the polymer matrix had sufficient time to vaporise all the polymer chains with weaker intermolecular bonding. As a result, the rest of the polymer chains with stronger bond strength and an intact structure required more heat energy to melt. Apart from that, when thermal ageing was prolonged to 8 days, T_m decreased due to the chain scissioning effects induced by thermal degradation at high temperatures. In addition to constant temperature thermal ageing at 100 °C, the trend of T_m of the samples subjected to condition Ramp-100 thermal ageing is shown in

Table 4. Melting temperature of samples subjected to condition Ramp-100 thermal ageing at various irradiation dosages.

Irradiation Dosage(kGy)	Melting Temperature (°C)
	2 Days Thermal Ageing	4 Days Thermal Ageing	6 Days Thermal Ageing	8 Days Thermal Ageing
0	108.57	108.17	108.74	109.05
100	110.71	110.5	110.95	111.2
200	111.73	109.72	111.43	111.23

. Similar to condition Constant-100, the T_m of polymers under ramp conditions increased with irradiation dosage due to the reinforcement of intermolecular bonds by the cross-linking effect. However, when the duration of thermal ageing was prolonged to 4 days, the resulting polymer samples experienced a decrement in T_m. This indicates that the structures of the polymer matrix were decomposed by chain scissioning mechanisms at elevated temperatures. After 4 days of thermal ageing operation, the T_m of all the polymer samples increased again. This was attributed to the rearrangement of molecules to form consistent and ordered structures in the polymer matrix. As a result, melting of the polymers was required to take place at a higher temperature, which indicates a higher T_m.

Apart from the analysis of T_m, the melting heat of the melted samples could be obtained from the integrated area under the curve melting heat of DSC thermograms as well. Figure 18 shows the melting heat of the ABS samples subjected to conditions Constant-100 and Ramp-100 thermal ageing at irradiation dosages of 100 and 200 kGy. Based on the curves, it can be seen that for the non-irradiated samples thermally aged under condition C-100, the area under the curve melting heat of the sample decreased with a prolonged duration of thermal ageing and reached a minimum value after 6 days. This indicates that the bonding spectrum of ABS molecules became narrower because, as the duration of thermal ageing was prolonged, the polymer chains with weaker intermolecular bonding were vaporised by the high-temperature heating effect. Therefore, the remaining polymers with stronger and larger molecule sizes resulted in a narrower bond spectrum. As thermal ageing was prolonged to 8 days, the area under the curve of the non-irradiated samples increased. This could be attributed to the cleavage of polymer chains induced by random scissioning mechanisms as a result of prolonged hours of thermal degradation (Freymond et al., 2022) [31]. Moreover, the area under the curve of the samples irradiated at 100 kGy was highest compared to the non-irradiated sample and sample irradiated at 200 kGy throughout 4 days of ageing. This implies that the amount of heat energy required to melt a sample of 100 kGy was relatively higher than that needed by other samples.

As thermal ageing further carried on, the area under the curve of both 100 and 200 kGy samples decreased progressively for 4 days. It was believed that a certain proportion of weak polymer chains had been vaporised due to the elevated temperature. However, similar to the non-irradiated sample, the area under the curve of the 200 kGy irradiation sample was found to increase after thermal ageing for 4 days and beyond. This could be explained by random scissioning of the polymer matrix, which contributed to a wider bond spectrum. In contrast, the sample of 100 kGy irradiation dosage had a smaller area under the curve after 4 days. This phenomenon shows that the vaporisation of weaker polymer chains occurred in conjunction with a prolonged duration of thermal ageing, ultimately narrowing the bonding spectrum.

On the other hand, it can be observed that the trend of the area under the curve for the non-irradiated samples thermally aged under condition Ramp-100 remained constant throughout 6 days of thermal ageing. This indicates that the area under the curve of the sample was independent at a short duration of thermal ageing (<6 days). However, when thermal ageing was prolonged to 8 days, the area under the curve of the sample decreased drastically, indicating a smaller bonding spectrum of molecules. This was attributed to the significant loss of weak polymer chains as a result of continuous vaporisation throughout the long duration of thermal ageing. For the irradiated samples of 100 and 200 kGy, the area under the curve increased and decreased respectively at a small scale from 2 to 4 days of thermal ageing. Nevertheless, as thermal ageing was further prolonged to 4 days and beyond, there were no significant changes in the area under the curve for both irradiated samples compared to the significant changes shown under condition Constant-100 thermal ageing. This phenomenon indicates that the bonding spectrum of the sample molecules remained unchanged as they were independent of the prolonged duration of thermal ageing.

4. Conclusions

Overall, the constant and ramp cyclic thermal processes have demonstrated degradation effects on ABS and its irradiated products.

(a)

For condition C-100 thermal ageing, the impact strength of samples decreased drastically with higher dosage and reached a minimum value of 54 J/m² at 250 kGy. For the tensile test, the value of tensile strength of samples thermally aged for 2 and 8 days decreased significantly (49–43 MPa) at high dosages (>100 kGy). This could be attributed to the altered chain scissioning process initiated by the heating effect and the prolonged duration of thermal ageing.

(b)

For the thermal ageing of condition Ramp-100, the trend of impact strength was similar to condition Constant-100. However, it could be concluded that the impact strength of condition Ramp-100 decreased at a lower degree (72–58 J/m²) compared to condition Constant-100 (73–54 J/m²). This was attributed to the strict and harsh heating environment of condition Constant-100 compared to the on-and-off heating of Ramp-100.

(c)

For samples thermally aged for 4–8 days, the effects of tensile strength versus various irradiation dosages were less significant during condition Ramp-100 thermal ageing as compared to Constant-100. This could mean that the effect of the thermal degradation and chain scissioning mechanisms was less significant during ramp condition thermal ageing as the heating effect was not fixed at a constant temperature.

(d)

The effects of irradiation dosage and thermal ageing on thermal properties were verified by DSC analysis. For the unaged samples, the melting temperature (106.24 °C) remained at a significantly low level. As the irradiation dosage increased up to 100 and 200 kGy, the melting temperature rose to 107.76 °C and 111.43 °C, respectively. This could be attributed to the strengthening of intermolecular bonds by the higher cross-linking network formed at higher irradiation dosages.

(e)

In conclusion, the cyclic thermal effect has shown less significant degradation of the ABS products compared to the constant thermal effect. This also provides constructive information that the service life of ABS products can be longer when the service temperature is in a ramp cyclic manner compared to the constant temperature environment, which tends to significantly reduce the service life of ABS.