Tailoring epoxy resin toughness for cryogenic conditions: An experimental approach

Abstract

This study explores the synergistic effect of polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and diglycidyl ether of bisphenol A (DGEBA) in reducing the inherent brittleness of epoxy resins. A 90/10 (90:10 proportion based on mass) composition of PC/ABS blended with DGEBA at 1.5 wt% modifier concentration demonstrated enhanced mechanical performance at both room and cryogenic temperatures. Comprehensive characterisation using FTIR, DSC, and optical microscopy confirms improvements in thermal stability and fracture resistance behaviour. Wear resistance is attributed to plastic deformation and tearing within the dispersed thermoplastic phase in epoxy matrix. For the case 90/10 blend with DGEBA, the impact strength of the modified DGEBA (m-DGEBA) matrix exhibited a maximum improvement of 20% at both room temperature (RT) and cryogenic temperature (CT). Rheology characteristics reveal an optimum viscosity rise of 0.2 Pa.s to 0.5 Pa.s, ensuring the optimal viscosity range of for infusion applications. This work presents a cost-effective and practical method for fabricating high-performance polymer composites suitable for advanced aerospace applications.

Keywords

Thermoplastic epoxy blending fracture rheology

Highlights

(1) The synergistic effect of polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) is utilised to induce ductility in DGEBA resins.

(2) The presence of hybrid thermoplastic (h-TP) blend in DGEBA effectively reduce the degree of crosslinking in cured DEGBA matrix.

(3) Composition 90/10 modified DGEBA exhibits least resistance to flow characteristics.

(4) 90/10 proves to be an effective hybrid thermoplastic (h-TP) composition in resisting micro-cracks generated from mechanical and thermal stresses.

(5) 90/10 proves as best composition in promoting the desired physical, thermal, and mechanical properties of modified DGEBA based composites at RT and CT.

Introduction

In the development of high-performance polymer composites for aerospace, military, transportation, and space applications, thermoset epoxy resins are widely employed. The exceptional strength of thermosets (TS) will ensure that fibre-reinforced composites have high stiffness and strength and a longer service life.^1,2 The primary problem with TS-based resins is their inherent brittleness, which results from their high degree of cross-linking.^3,4 In addition, polymer composites with TS as the matrix will have the lowest recycling and reuse preference. This is primarily because it is difficult to extract the reinforcement from the TS matrix after their service life. Consequently, thermoplastic (TP) resin alternatives are adopted despite their inferior strength and stiffness compared to composites based on TS matrix. To increase the usability of TS resins for composite development, a vast array of modification techniques is being investigated globally. To reduce the brittle characteristics and cracking nature of TS resins under extreme external load conditions (mechanical and thermal), a variety of modification schemes involving fibre,⁵ matrix,^6–8 or both fibre and matrix modifications^9,10 are employed, along with various types of modifiers ranging from nanoparticles to surface-modified long fibre. The development of fibre surfaces through chemical, mechanical, or electric deposition treatment involves the removal or addition of specific groups on the fibre surface, resulting in a larger surface area for the TS resins to adhere to, thereby enhancing toughness and fracture resistance.¹¹ But the modification techniques are rather difficult and expensive. For the present study, however, we are concentrating on techniques involving direct modification of TS resins. Therefore, fibre modification techniques can be a topic for another study in the future. The direct modification of TS resins with a TS or TP modifier is highly considered due to its simplicity, cost-effectiveness, and ease of processing. As modification strategies, solvent mixing and melt-mixing of TS or TP modifier into selected TS resins are utilised commercially. For the purpose of dissolving the modifier, excessive amounts of organic solvents with a high concentration that are specific to the modifier are used for solvent mixing. The dissolved modifier is then added to the selected TS resin, from which the solvent is extracted by heating the mixture at a constant temperature that is close to the flash point of the chosen solvent.¹² As a result, the process is laborious, polluting, and fraught with danger. In addition, the presence of unremoved solvent portions in the mixture leads to undesirable void formation during curing, resulting in failure zones within the bulk that make the material susceptible to immediate failure.^13,14

In the process of melt-mixing, the TP modifier is melted and mixed with the TS resin at a constant temperature (temperature higher than the glass transition temperature of selected TPs) via a direct and straightforward mechanism. Melt-mixing ensures processing simplicity and the flexibility to combine multiple TP modifiers with TS resins.¹⁵ The selection of TP modifier specific to the TS resin should take into account the chemical structure similarity, the modifier content, and the modified TS resin’s morphology. As modifiers for Diglycidyl Ether of Bisphenol A (DGEBA) type TS resin, numerous types of TP modifiers such as polyetherimide (PEI), polyimide (PI), polyethylene (PE), polycarbonate (PC), polybutylene terephthalate (PBT), and acrylonitrile butadiene styrene (ABS) are utilised.¹⁶ The addition of TP component to TS resin produces locking of dispersed TP polymer networks within cured TS matrix. The combination of locking mechanism similar to the formation of interpenetrating polymer networks (IPN) and the inherent ductility of TP has been reported to increase the fracture toughness and microcrack resistance of carbon fibre reinforced polymer (CFRP) composites. However, in both solvent and melt-mixing, the improvement in results is limited to the peak concentration of the TP modifier.¹² This limiting concentration of TP modifier is observed to be significantly more crucial at cryogenic temperatures.¹² The enhanced thermal and mechanical properties of modified TS (m-TS) resins are followed by a maximum increase in viscosity. The increase in viscosity of m-TS resins will result in increased resistance to flow. The reported viscosities of m-TS resins are well above 10 Pa.s,¹⁶ making them incompatible with resin infusion methods (RIM) (permissible resin + hardener viscosity range of 0.2 to 0.5 Pa.s).¹⁷ In order to bring the viscosity of the m-TS resins closer to the permissible range for RIM, a higher concentration of hardener must be added. This will result in a higher degree of cross-linking reaction, which will increase the brittleness of the cured m-TS resin system, a characteristic that may be undesirable for many applications.¹¹ In order to address this issue, modifier concentrations for TS resins must be determined with special care. This will aid significantly in controlling the viscosity increase of m-TS resins.

The utilisation of multiple TP modifiers can be exploited in boosting the performance of modifiers with TS resins. In the manufacturing and transportation industries, the synergy between multiple TPs is extensively researched and implemented to enhance the performance of machined parts.^9,15,18,19 One specific example is the synergy exhibited by the PC/ABS blend system, which is widely utilised by manufacturing plants because the blends exhibit a significant improvement in performance when compared to the results produced by each material alone.¹⁹ The fracture and micro-crack resistant capabilities of PC/ABS/DGEBA-based CFRPs at both room and cryogenic temperatures have been confirmed by our previous detailed studies.⁹ This study investigates the effect of low weight percentage (1.5 wt% of DGEBA) PC/ABS modifiers in DGEBA on the rheology, heat flow, curing, and cryo-toughening characteristics of modified DGEBA (m-DGEBA) at room temperature and cryogenic conditions. The current research is significant because it may aid in the comprehension of the impact that the concentrations of PC and ABS have on DGEBA. This impact may be positive or negative synergism, leading to an enhancement or degradation of the desired properties. An aspect of novelty in this study is the improved comprehension of the magnitude of the modifier effect at the 1.5 wt% concentration (which remains constant for the entire modifier content). The objective was to furnish insight into the rheological and curing characteristics subsequent to modification, which could aid in the determination of modifier compositions at a particular concentration, thereby enabling the preparation of composites through resin infusion or manual layup.

Materials and methods

Materials

In this work, virgin PC and ABS are chosen as modifiers for DGEBA resin. The materials and their sources are listed in Table 1, which was previously utilised in our previous published works.^9,15,18

Table 1.

Technical details of TS and TP materials used for modification via melt-mixing approach.

Sl. No	Material	Specification
1	Epoxy Resin	Diglycidyl ether of bisphenol A (DGEBA) (YD 128), epoxide equivalent weight (EEW) of 185 to 194 g/eq, Aditya Birla Chemicals Limited, Thailand
2	Hardener	Modified cyclo-aliphatic amine (TH 7301), Aditya Birla Chemicals Limited, Thailand
3	Thermoplastic modifiers
	(a) Polycarbonate (PC)	Virgin quality, Makrolon 2856, Covestro AG, Germany
	(b) Acrylonitrile butadiene styrene (ABS)	Virgin quality, Cycolac MG 47F, Sabic, Saudi Arabia

Methods

Resin modification using melt-mixing

For resin modification, uncured DGEBA resin with weighed compositions of PC and ABS as mentioned in Table 2 is taken in a prototype mechanical stirrer with temperature-controlled crucible developed at Space Technology Laboratory, Thangal Kunju Musaliar College of Engineering, India.

Table 2.

PC/ABS composition used for melt-mixing and observed modified DGEBA colour change.

Sl.No.	Specimen code	DGEBA (g)	PC (g)	ABS (g)	Processing time (minutes)	Colour
1	u-DGEBA	100	0	0	0	Transparent
2	PC100 m-DGEBA		1.5	0	20	Slight yellow
3	90/10 m-DGEBA		1.35	0.15		Moderate yellow
4	10/90 m-DGEBA		0.15	1.35		Moderate yellow
5	ABS100 m-DGEBA		0	1.5		Moderate yellow

The mixture is maintained at 180°C and mechanically stirred at 1440°rpm continuously until the entire TP portion has melted and been thoroughly mixed into the hot resin. The m-DGEBA and u-DGEBA resins are as shown in Figure 1

Various compositions of PC/ABS parts (PC100, 90/10, 10/90 and ABS100) were selected for modifying DGEBA resins via melt-mixing. The composition of PC/ABS ranges from 100% PC, 90% PC and 10% ABS, 10% PC and 90% ABS and 100% ABS concentrations in the 1.5 wt% of TP modifier content. The compositions and processing time for modification of DGEBA is represented in Table 2. The TP modifiers, PC and ABS are available as 2 mm to 3 mm pellets that, when directly melt-mixed with DGEBA, require approximately 45-60 minutes.¹⁵

To expedite the procedure, the pellets were converted into flake form by pressing them in a constant temperature mechanical compression unit at 200°C under a pressing load of 10 kN for a duration of 5 minutes. The reduction in thickness, resulting in an increase in surface area, facilitates the melting and mixing of TP content into DGEBA at 180° C in less than 20 minutes. To ensure a minimal increase in viscosity of m-DGEBA resins after melt-mixing, the maximum TP modifier content to be melt-mixed into the resin is set as 1.5 wt% of DGEBA portion. In addition, this is the maximum concentration of TP in DGEBA necessary to ensure an improvement in mechanical properties, as reported in previous studies.^12,15

Characterisation studies to determine the synergistic effect of hybrid TP modifiers in DGEBA

Utilising the synergistic effect of the hybrid TP effect, the present work seeks to modify DGEBA resins with minimal modifier content. On m-DGEBA resins, FTIR, DSC, and rheology tests were conducted along with exothermal profiling of curing, and toughness evaluation to determine the synergistic effect of the hybrid PC/ABS/DGEBA blend. To confirm the chemical effects of the hybrid TP modifiers with DGEBA, FTIR, and DSC measurements were performed on cured m-DGEBA samples and compared to the unmodified DGEBA (u-DGEBA). The m-DGEBA cured samples were scanned using the attenuated total reflection (ATR) contact sampling method with the help of the Perkin Elmer spectrum two FT-IR spectrometers. The interferograms are generated through the signal averaging of 28 scans with a resolution of 4 cm⁻¹. The spectra are derived by calculating the percentage transmittance in relation to the wavenumber.¹⁵ For DSC, a heating rate of 10 K/min using the DSC204F1 Phoenix from NETZSCH between 30°C and 240°C was used.¹⁵ Uncured m-DGEBA and u-DGEBA were tested in an Anton-Paar Rheometer, MCR 102 series, at a constant strain rate of 500/s over the temperature range of 30°C to 110°C to determine their rheology properties in rotational mode. To determine any changes in curing reaction and curing time, m-DGEBA and u-DGEBA were mixed with hardener at a ratio of 2:1 by weight (two parts resin to one part hardener) and then poured into insulated acrylic moulds with J-Type thermocouples and allowed to cure under vacuum at room temperature, as depicted in Figure 2(b). The cure reaction temperature of all cases is simultaneously recorded, and the data can be used to determine the reaction rate and predict the effect of hybrid blend on cross-linking nature as shown in Figure 2(a). For the purpose of determining the cryo-crack resistance of m-DGEBA resin, 5 mm thick, 5 cm in diameter disc specimens were cryo-cycled five times in liquid nitrogen (LN2). In the cyclic treatment process, samples at 30°C are taken to cryogenic temperatures by immersing them in LN2 which is confirmed using a T-type thermocouple. After this, the sample is taken out from LN2 to return back to 30°C. This constitutes a cryo-cycle. In this study, 5 similar cycles and their corresponding before and after high resolution images were used to analyse the cracks.

To evaluate the impact resistance of cured m-DGEBA, five 10 cm × 1.5 cm × 1 cm specimens of each modification were prepared for the Charpy test as per ASTM D-256. For this, a pendulum of 30 kg⋅m energy is used for determining the impact strength of cured samples. To further examine the resistance of m-DGEBA to cracking after cryo-treatment, the samples at 30°C were cryo-treated in LN2 by immersing the specimens in liquid nitrogen for 2 hours prior to the Charpy Test to allow them to cool to 77 K. The samples were immediately taken out from liquid nitrogen and fractured within 5s. The fracture surfaces were scrutinised for better understanding the effect of PC and ABS modifiers in DGEBA. The heat flow resistance characteristics were tested by understanding the heat gain characteristics of cured DGEBA after modification using Hot Disc TPS 500S following ISO 22007-2 standards. For this rod samples of dimension 10 mm long x 5 mm wide circles are prepared and tested in a Kapton 5645 F2 disc type measuring system with conditions 60 mW power output for a time of 40s at 33°C. To determine the wear resistance effects of modified epoxy composites, surface morphology of wear specimens post pin-on-disc experiment as per ASTM G 99 was opted.²⁰ Following the conclusion of the pin-on-disk studies, the worn surfaces of the prepared composites for which the maximum load and sliding distance are then examined using an optical microscope at 100x. This microscope enables thorough inspection of the wear tracks and wear debris produced during tribological testing. The study conducted with the metallurgical microscope offers valuable information regarding the worn surface in the microstructure, processes of wear for different test conditions.

Results and discussion

Determination of bonds dissolved or formed in m-DGEBA

This work involves modifying DGEBA resins to reduce their brittleness at room temperature (RT) and cryogenic temperatures (CT). To improve the fracture toughness characteristics of DGEBA, the flakes of TP modifiers PC, ABS, and PC/ABS are melted with DGEBA at 180°C. As depicted in Figure 3, FTIR spectroscopy of m-DGEBA and u-DGEBA is compared to confirm the nature of the modification scheme (chemical or physical) involved. The u-DGEBA exhibits characteristics between 3300 cm⁻¹ and 3570 cm⁻¹ that confirm -OH stretching, whereas a slight peak between 2750 cm⁻¹ and 2910 cm⁻¹ may be attributed to -CH stretching.¹⁵ Melt-mixing of PC into DGEBA at all PC/ABS/DGEBA hybrid blend compositions exhibits higher peak intensities at 2850 cm⁻¹ to 2910 cm⁻¹,^21,22 whereas ABS100 composition does not exhibit this trend.

In addition, the intensities of the additional characteristic peaks in the range of 650 cm⁻¹ to 1450 cm⁻¹ observed in PC100 m-DGEBA appear to decrease as the concentration of PC in PC/ABS composition decreases. The significant characteristic peak confirming ABS in DGEBA is observed at 1603 cm⁻¹ and 2205 cm⁻¹, confirming -C = C stretching in butadiene and -CN stretching in acrylonitrile, respectively.²³ As observed in 90/10 m-DGEBA and completely absent in PC100 m-DGEBA, the intensity of these peaks decreases with decreasing ABS concentration. However, the absence of a peak at 913 cm⁻¹ in all m-DGEBA indicates the full participation of epoxide groups in the curing reaction with the selected hardener.¹⁵ The absence of a peak at 913 cm⁻¹ further verifies that complete cross-linking has been achieved, ensuring the maximum achievable strength after the curing reaction²⁴ has been attained. This property ensures that modification has not interfered with the curing process and that m-DGEBAs will be capable of undergoing a complete curing reaction to achieve its highest possible strength and stiffness.²⁵

In all of the characteristic curves shown in Figure 3, 90/10 m-DGEBA exhibits the greatest decrease in peak intensities, indicating that it has the strongest role in chemically modifying DGEBA resins by bond dissolution or formation compared to other selected hybrid blend compositions.^12,15 The partial miscibility of PC in DGEBA observed in earlier studies^12,25,26 may be the reason for the chemical modification of DGEBA observed in this study, which is much more pronounced with the 90/10 hybrid blend system. DSC analysis is performed to further confirm the chemical modification and the reactive effects of hybrid blends, as described in the following section.

Evaluation of heat flow and thermal characteristics of m-DGEBA

Figure 4(a) depicts the heat flow characteristics of m-DGEBA samples. The presence of virgin PC (v-PC) and virgin ABS (v-ABS) were confirmed using the DSC heat flow characteristics from the exhibited slight change in their baseline (glass transition temperature, Tg) observed at 149°C and 107°C, respectively.^23,27 From the heat flow characteristics, we can observe the steepness of peak for modified DGEBA compared to unmodified DGEBA which may be attributed to the presence of PC or ABS in the matrix. The DSC area and T_g obtained after each modification is different even for the same concentration of thermoplastic modifier (1.5 wt% of DGEBA) used. Out of the different PC/ABS compositions used to modify DGEBA, the composition with a ratio of 90/10 exhibits the highest DSC area, as depicted in Figure 4(a). Furthermore, it is evident that when the concentration of ABS in the modifier composition exceeds 10%, there is a decrease in DSC area, with the lowest result observed for a composition consisting solely of 100% ABS. The observed expansion in DSC area for PC100 can be attributed to the supplementary thermal energy generated by the crystallisation of PC in the modified DGEBA during the curing process. This phenomenon can also be related to the amplified chain mobility subsequent to modification, which in turn leads to a decrease in the cross-link density of the matrix following the curing process. As observed in previous works, the addition of ABS into DGEBA results in reactant dilution.²⁸ The decrease in area is contingent upon the concentration of ABS, as depicted in Figure 4(a).

Raising the concentration of ABS in the modifier composition for DGEBA leads to a greater dilution of the active reaction zones, resulting in a decrease in the release of exothermic energy. In the context of ABS100, the dilution effect of ABS diminishes the active reaction zones in epoxy during a curing process, leading to a reduced increase in DSC area compared to PC100. When PC and ABS are used in combination as a hybrid composition, the area is found to be greater than that of their individual compositions and unmodified cases. This can arise from the synergistic outcome of the transesterification reaction between PC and DGEBA, as well as the tendencies of ABS to undergo phase separation and dilution with DGEBA. The variation in observed T_g values after modification of DGEBA may be attributed to the strengthening or weakening effect of PC/ABS composition in DGEBA. An increase in T_g may be a result of reinforcing effect of PC/ABS modifier in DGEBA and is highest visible for 90/10 composition of PC/ABS in DGEBA.

Figure 4(c) depicts the thermal resistance characteristics of m-DGEBA with respect to time. The highest temperature increase is recorded for u-DGEBA as shown in Figure 4(c). In contrast, PC100 and ABS100 m-DGEBA are observed to reduce temperature rise by 17.1% and 31.9%, respectively. This is because the dispersed TP modifier in the DGEBA matrix acts as insulation zones. Among the hybrid immiscible blends, it has been determined that 90/10 provides greater resistance to heat flow than 10/90 m-DGEBA (59.0%). In the case of 90/10 m-DGEBA, the distribution of partially soluble PC and immiscible ABS in DGEBA occurs by preserving a more uniform heterogenic morphology, as previously reported.²⁹ Thus, the PC portion tends to form a co-continuous phase in a continuous DGEBA matrix on which low concentrations of ABS form droplet morphology.⁸ These droplets in a 10% ABS concentration have a tendency to impart positive properties to the matrix, such as increased load bearing and thermal stability by reducing the void content in modified matrix.

From Figure 4, we can observe that the area, T_g and thermal resistance values are higher for 90/10 modified DGEBA compared to all other cases studied. FTIR analysis confirms that these modifications have not resulted in any instances of incomplete curing. The dilution and induced flexibility of bonds for modified DGBEA is to be further analysed to determine the extent of modifier effect in enhancing the thermal, flow, and mechanical properties of DGEBA resin.

Rheology characteristics of m-DGEBA

When the PC/ABS hybrid portion was melt-mixed into DGEBA at 180°C and then cooled to RT, the m-DGEBA resins exhibited greater resistance to flow than the u-DGEBA resins. The resistance to flow observed for m-DGEBA was significantly different for selected modifier compositions.³⁰ Consequently, it was deemed essential to determine the extent of the effect of PC and/or ABS concentration on the viscosity of PC/ABS portion in m-DGEBA resins. Figure 5(a) depicts the viscosity versus temperature characteristics obtained from the MCR102 rheometer. As expected, all m-DGEBA resins exhibit a greater increase in viscosity than u-DGEBA resins, and this increase is significantly greater at 30°C. As temperature rises, the viscosity difference between u-DGEBA and m-DGEBA resins tends to diminish and achieve rather closer viscosity values (2 Pa.s to 4 Pa.s). At temperatures between 40°C and 50°C, the viscosity of resins ranges from 1. 8 Pa.s to 4 Pa.s. From the viscosity values observed in Figure 5(b), m-DGEBA resins can be infused at temperatures between 40°C and 50°C since they exhibit rather similar viscosity values when compared with u-DGEBA due to increased chain mobility at higher temperatures.³¹ According to previous research,³² PC/ABS exhibits exceptional synergistic effects in the case of 90/10 (90% PC concentration and 10% ABS concentration) by utilising the effect of PC’s strength and stiffness and ABS’s flowability and processability.¹⁹ Figure 5(a) demonstrates that, for the same reason, 90/10 m-DGEBA exhibits the lowest viscosity increase when compared to PC100 m-DGEBA. The 10% ABS concentration in the hybrid blend modifier improves the flow properties of m-DGEBA, resulting in a lower viscosity increase providing it higher flow capabilities. This might be partially attributed to the morphology of 90/10 modified DGEBA with co-continuous PC phase enclosing ABS part dispersed in continuous DGEBA phase.⁸ Considering the effect of PC or ABS concentration on the viscosity increase of m-DGEBA, it is evident that as the ABS concentration in a PC/ABS composition increase, so does the viscosity increase.

In the case of a 10/90 hybrid PC/ABS composition, however, the observed viscosity is slightly greater than that of ABS100 m-DGEBA. This may be due to the tendency of immiscible ABS portions dispersed in DGEBA continuous phase to form larger discontinuities and voids within the PC co-continuous phase.¹⁹ As a result, the ABS rich zones in the matrix tends to act on their own restricting the uniformity in flow exhibited by the continuous and co-continuous phases in 10/90 m-DGEBA resulting in higher resistance to flow. This highly heterogeneous morphology, with very low synergistic effect as described in our previous publications,^15,18 may be partially responsible for this undesirable characteristic of the 10/90 blend,³² which results in a negative synergistic effect with DGEBA resin making the 10/90 hybrid blend unsuitable to use with DGEBA. In the case of ABS100 m-DGEBA, however, the high concentration of ABS does exhibit coalescence tendencies to form agglomerated zones, resulting in a heterogeneous morphology,^15,29 but does not significantly affect the morphology in an adverse way to resist the flow in comparison to 10/90 m-DGEBA.

Figure 5(b) displays the viscosity of m-DGEBA resins at different temperatures (30°C, 40°C, 50°C, 60°C, and 70°C). For RIM, a resin/hardener mixture is utilised between 30°C and 40°C with a viscosity range of 0. 2 Pa.s to 0. 5 Pa.s.¹⁷ In the specified temperature range of 30°C to 40°C, 90/10 m-DGEBA displays the least viscosity increase over the infusion temperature range compared to all other m-DGEBA resins. Over 30°C to 40°C, viscosity characteristics of m-resin/hardener mixture exhibit a similar trend, with viscosity values of 0. 25 Pa.s to 0. 48 Pa.s for 90/10 and PC100 m-DGEBA/hardener mixtures, respectively using MCR102 rheometer. The observed viscosity is well within the acceptable range for use in the RIM process. In comparison to PC, ABS, and other PC/ABS combinations, the 90/10 hybrid composition of PC/ABS is renowned for its strength and processability [90/10 blend flow citation]. This characteristic of 90/10 is inherited by DGEBA resins, resulting in minimal viscosity increase in comparison to other modifications as confirmed in Figure 5. Therefore, the 90/10 hybrid TP modifier is the most suitable modification for the selected resin system (DGEBA) with the least viscosity increase over the 30°C to 40°C temperature range. Among the selected hybrid TP modifiers, the ability of 90/10 to chemically modify DGEBA while retaining good flow characteristics makes it the superior modifier among the selected hybrid TP modifiers.

Cure reaction characteristics of m-DGEBA

For this study, the m-DGEBA resins undergo a complete cross-linking curing reaction with the selected hardener as confirmed using FTIR (Figure 3). To determine whether there is a change in curing time or reaction temperature, a study was designed specifically for this purpose. In this section, properties such as curing time and temperature of reaction were recorded and analysed as shown in Figure 6. Figure 6 displays the average curing temperature obtained from the DAQ using J-type thermocouple housed within acrylic moulds. In all instances, the curing reaction occurs in three stages: a liquid phase, a gel phase, and a solid phase as shown in Figure 6. As depicted in Figure 6, the cure characteristics shift to the right. This shift has been found to be directly proportional to the ABS content of hybrid PC/ABS modifiers. Also, the peak temperature observed during the curing reaction decreases and shifts towards the right as the ABS concentration increases. The highest observed curing reaction temperature is for u-DGEBA resin (38.1°C). The temperature then decreases gradually with increasing ABS concentration, reaching a minimum of 35.2°C for the ABS100 m-DGEBA system. We can conclude from this study that ABS has the ability to reduce the cure reaction temperature, which is indirectly related to the chemical reaction rate.²⁵ Notably, the curing temperatures of the hybrid blends 90/10 and 10/90 are very close (36.5°C and 36.3°C, respectively). Intriguingly, the peak curing temperature, which denotes the complete participation of epoxy monomers in the curing reaction, exhibits a rightward shift. This is due to the decreased concentration of effective monomers participating in the curing reaction.

Consequently, the time required to reach peak cure temperature varies from 40 minutes to 56 minutes for u-DGEBA and ABS100 m-DGEBA, respectively. This will provide the user with additional time to work with the m-resin/hardener system, making it suitable for complex projects. Figure 3 depicts chemical reactions that occur during the modification of DGEBA, resulting in bond dissolution. The reduction in cure temperature and increased cure time is further evidence of this reaction.²⁵ The phenomenon of bond dissolution may have reduced the effective epoxy monomer content that could participate in the curing reaction, thereby decreasing the degree of cross-linking during the curing reaction. Higher cross-linking is theorised to correlate with the brittleness of epoxy.³³ This is further supported by the lowered temperature of the curing reaction confirming the hybrid TP modifiers capability in reducing the cross-linking rate of m-DGEBAs. This might allow m-DGEBAs to achieve the necessary fracture toughness characteristics, which can be used to develop high-performance composites for aerospace applications.

Cryo-crack resistance performance of m-DGEBA

The primary objective of this study is to determine the effectiveness of hybrid PC/ABS modifiers in enhancing the crack resistance of DGEBA at RT and CT. The impact strength characteristics of hybrid PC/ABS m-DGEBAs at RT and CT are depicted in Figure 7(a) and (b). Figure 7(a) displays the average Charpy impact strength of m-DGEBA specimens at RT and CT. In this study, particular care was taken to maintain the specimens at cryogenic temperatures (CT) during testing to ensure the cryogenic condition effects (near cryogenic temperatures of specimens at the time of impact). This was accomplished according to reported studies conducted by Yuxin et.al.¹² The impact strength of PC/ABS was found to be highest for 90/10 composition as observed by Chiang et.al.³⁰ This may be due to the ability of the 90/10 modifier to resist cracking and thereby increase the impact strength of DGEBA under CT conditions while simultaneously boosting its strength.³⁴ All modifiers are capable of increasing the impact potency of DGEBA at RT and CT, with 90/10 m-DGEBA producing the best results. From Figure 7(a), it is evident that the 90/10 hybrid blend outperforms PC100, demonstrating the positive synergistic effect of the 90/10 blend.³² This can be attributed to the better cooperative behaviour of 90/10 blend with DGEBA as confirmed with DSC. The decrease in strength observed between 10/90 and ABS100 confirms the negative synergistic effect of the hybrid blend with DGEBA.

The presence of a lower ABS concentration results in distributed droplet morphology in a continuous (DGEBA) and co-continuous (PC) phase. The droplet portions of ABS act as load damping zones, resulting in enhanced strength characteristics of 90/10 m-DGEBA at RT and CT [10% ABS reason] due to the droplet portions of ABS acting as load damping zones. But, as the size of distributed droplets grows due to nucleation effects, the formation of voids in the morphology is greater resulting in unpleasant/reduced results as observed with ABS dominant m-DGEBAs (ABS100 and 10/90 m-DGEBAs).¹⁹ The longer gel time followed by decrease in apparent cross-linking density might be the cause for lower impact strength in ABS 100 m-DGEBA both at RT and CT.²⁵ Therefore, in the present work, PC-dominant hybrid blends are observed to provide DGEBA with the greatest possible increase in strength and fracture resistance at RT and CT conditions. This makes them a better suitable modifier for this specific work when compared to ABS dominant modifiers.

For additional confirmation of the cryo-crack resistance mechanism, m-DGEBA specimens were subjected to multiple cryo-cycles and the cracks were analysed in detail, as shown in Figure 8. Figure 8(a) to (c) illustrate the various stages of m-DGEBA samples during the cryo-cycling experiment. Figure 8(b) and 8(c) depict samples after the initial and fifth cryo-cycling, respectively. It is evident from Figure 8(b) that specimens exhibit cryo-cracking after the initial cryo-cycling and tend to grow more at the conclusion of the fifth cryo-cycling. The cracks exhibit a deviation in growth pattern and criticality, confirming that the matrix’s predominant brittleness has changed. Figure 8(d) and (e) depict monochromatic high-resolution images of the upper and lower surfaces of specimens following the fifth cryo-cycling. To further determine the cryo-crack resistance capabilities of hybrid TP modifiers in DGEBA, the crack paths formed after the 5^th cryo-cycling was criticality analysed. The red and blue lines in Figure 8(d) and (e) represent continuous and discontinuous crack profiles, respectively. The sections highlighted in yellow represent the severity of cracks in bulk. The u-DGEBA matrix displays fractures in continuous patterns with no path deviations. This pertains to the predominant brittleness exhibited by the u-DGEBA matrix as a result of its poor fracture resistance properties. In the case of PC100 m-DGEBA, there are no significant deviations, but there are fewer discontinuities in crack paths. This is due to the predominant brittleness of PC content at temperatures below −40°C.¹² ABS100 m-DGEBA has smoother crack paths and fewer discontinuities than u-DGEBA, rendering it unsuitable for cryogenic applications. In the case of hybrid PC/ABS modifiers, 90/10 yields the greatest improvement in cryo-crack resistance properties due to reduced cross-linking density.³³ This is supported by the existence of numerous discontinuous crack paths. This is a result of the cooperative nature of molecules in 90/10 m-DGEBA, as observed by DSC results, and the inherent ductility of DGEBA, as reported in our earlier work.^9,15,18 The higher cooperation of molecules in 90/10 m-DGEBA enables the modified matrix to effectively deal with transfer of thermal stresses resulting in lower stress concentrations. As a result, the energy required to initiate cracks are limited resulting in crack resistance capabilities of 90/10 m-DGEBA. The crack path deviation and discontinuities are the result of ductile tearing and crack path deviation at plastic zones dispersed throughout the brittle matrix.^9,15 In contrast, the crack paths of 10/90 m-DGEBA exhibit profiles similar to ABS100 m-DGEBA with fewer discontinuities. The formation of phase-inverted morphologies is commonly observed in ABS dominant concentrations. Higher ABS concentration results in nucleation, which produces larger droplets in the morphology.¹⁹ This results in the formation of voids and discontinuities in the matrix, leaving zones for stress localisation. Ultimately, the voids serve as crack initiation zones and provide crack propagation pathways. Based on these findings, the 90/10 hybrid blend provides DGEBA with sufficient cryo-crack resistance characteristics that can be utilised to develop high performance hybrid polymer composites for space and cryogenic applications.

Morphology aided toughness evaluation of m-DGEBA composites

The ability of 90/10 m-DGEBA to resist cracks and enhanced temperature resistant characteristics makes them a suitable material for the development of high-performance composites. In this section, the ability of 90/10 m-DGEBA in resisting wear is slightly exhibited by performing pin-on-disc experiment in prepared composites. This study was inspired as a result of our previous published article,⁹ where we explored the synergistic effect of 90/10 in enhancing the toughness of carbon fibre reinforced composites. But it was required to determine the extent of effect of 90/10 in tribological characteristics compared to unmodified composites. Figure 9(a) and (b) show the wear surfaces of unmodified and 90/10 modified composites, magnified by 100 times. Figure 9(a) displays numerous deep scratches without any apparent limitations or restrictions. This could be attributed to the inherent fragility or brittle nature of the unmodified epoxy matrix. Consequently, the surface experiences increased wear, resulting in the formation of cleaner and more visible scratches with no sign of resistance. Figure 9(b) displays a rough morphology where dispersed portions of PC and ABS are visible within the continuous epoxy matrix as teared morphologies. The inclusion of scattered segments of PC and ABS within the brittle epoxy continuous matrix tends to inherit a plastic characteristic to the predominant brittle structure.^15,18 Consequently, the wear is limited to several areas that are marked by dull scratches and less noticeable scratches on the surface. The majority of the force is absorbed by deforming the plastic particles dispersed in the brittle epoxy matrix, resulting in reduced wear and increased scratch resistance. Therefore, it is evident that using a 90/10 modifier composition with DGEBA greatly enhances the toughness and wear resistance of composite surfaces, making them a promising material for the advancement of high-performance polymer composites.

Conclusion

This study sheds light on the synergistic effect of hybrid PC/ABS modifier in enhancing the fracture resistance properties of DGEBA matrix.

• Utilising the synergistic effect of a 90/10 blend with DGEBA, the impact strength of the m-DGEBA matrix was increased by a maximum of 20% at RT and CT.

• The curing characteristics also indicate an increase in peak curing time from 40 minutes to 56 minutes, which makes the modified resins more suitable for resin infusion moulding (RIM) processes.

• Additionally, 90/10 m-DGEBA exhibits the lowest viscosity increase compared to other m-DGEBAs, ensuring the optimal viscosity range of 0.2 Pa.s to 0.5 Pa.s for infusion applications. Modifying the resins to achieve a longer infusion time and a controlled viscosity range makes it simple to use with complex design shape.

• The inherent plastic zones within the predominant brittle matrix of DGEBA cause deviation of crack paths and higher fracture energies, resulting in cryo-toughening.

The efficacy of melt-mixing’s synergistic effect in modifying DGEBA for developing high performance polymer composites is confirmed by the decreased cross-linking density, increased fracture toughness, and cryo-crack resistance.

Footnotes

Acknowledgements

This research was made possible by a scholarship from the National Doctoral Fellowship (NDF) Scheme, 2018 administered by the All India Council for Technical Education (AICTE). We would like to thank SIF NIT Trichy for assisting us in obtaining the DSC results utilised in this investigation. The authors would like to thank the Ministry of Higher Education Malaysia and Universiti Teknologi Malaysia for supporting this work by providing the UTM Fundamental Research Grant (UTMFR) (Q.J130000.3824.22H91). We would also like to thank the Space Technology Laboratory at TKM College of Engineering for providing the facilities necessary to complete this project.

Author contribution

Aravind J: Writing – review and editing, writing – original draft, methodology, investigation, data curation, and conceptualisation. Manu M: Writing – review and editing, writing – original draft, methodology, investigation, conceptualisation. Sanal Mohammed B: Writing – original draft, investigation, formal analysis, data curation. K.E. Reby Roy: Supervision and data curation. Mubarak Ali M: Supervision, data curation. Mohd Yazid Yahya: Software, resources, and funding acquisition. Anukrishna G S: Investigation and formal analysis.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Krishnan Kutty Elsy Bai Reby Roy

Data Availability Statement

The data that support the findings of this study, including raw experimental data, are available from the corresponding author upon reasonable request. *

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