Solution blending, casting, thermo-mechanical properties and surface morphology of epoxy resin–polycarbonate blends

Combining epoxy and polycarbonate through blending yields a material that amalgamates the favorable attributes of each polymer. The applications of blends comprising epoxy and polycarbonate, especially in the fields of aerospace automobile components, medical devices, electronics and electrical enclosures, coatings and adhesives, etc (Khan et al., 2024; Equbal and Sood, 2015; Equbal et al., 2013). Polymer blending is one of the effective ways to obtain materials with specific properties. Blending improved mechanical properties, chemical properties and processibility also formed a high-performance blend from synergistically interacting polymer (Gama et al., 2019). The unique sets of properties of the individual polymer in a multicomponent system are the basis of polymer blending. “Mixing together of two or more different polymers or copolymers is known as blending.” The major advantages of blending are high impact strength easy in processibility high tensile strength, high modulus/rigidity, good heat deflection temperature, better flammability and better solvent resistant, high thermal stability, excellent dimensional stability good elongation (Khan et al., 2022). As one of the most widely used thermoset materials, epoxy resins have special chemical characteristics compared with other thermosetting resins: no byproducts or volatiles are formed during curing reactions, so shrinkage is low, and degree of cross-linking can be controlled. But cured epoxy systems have one main drawback: their considerable brittleness, which shows poor fracture toughness and low impact strength. This inherent brittleness has limited their application in fields requiring high impact and fracture strengths, such as reinforced plastics, matrix resins for composites, and coatings (Singh et al., 2022; Polymer Science and Technology Plastic, 2011; Handbook of Epoxy Blends, 2017). Polycarbonate (PC) plastic is a light weight, high-performance plastic, possesses a unique balance of toughness, dimensional stability, optical clarity, high heat resistance and excellent electrical resistance, outstanding resistance to long term static stress; their creep characteristics are generally better than most of other thermoplastics, coupled with impact resistance makes polycarbonate resins stiff and hence appropriate for structural applications, Exceptionally good impact strength with comparatively high elongation at break during tensile testing, indicates a degree of ductility (Handbook of Epoxy, 2022; Chen et al., 1992; Rao et al., 2002; Priya and Rai, 2006; Rong and Zeng, 1997; Park et al., 1999). Polycarbonate is one of the toughest materials among the unmodified plastics. It has a number of desirable properties viz. rigidity up to 140^οC, toughness up to 140^οC, excellent transparency, very good electrical insulation characteristics, virtually self-extinguishing and physiological inertness (Farooq et al., 2020; Harni et al., 1999; RajuluReddy et al., 2002; Vishu, 2007; Naveen et al., 2000). Enhancing the toughness of epoxy materials is achieved by altering the base epoxy resins. This involves integrating a secondary component into the epoxy matrix through physical blending or chemical reactions. Proper dispersion of these modifying elements throughout the epoxy enhances fracture energy and toughness significantly (Khan et al., 2020; Mimura et al., 2000; Khan and Hussain, 2020; Khan and Faheem, 2023). Similar kinds of studies are performed by a few researchers. Ahmetli et al., (2012) investigated epoxy resin/polymer blends for improvement of thermal and mechanical properties. Fatty acid waste was used to create polymers with epoxy or unsaturated ester groups. The polymers were made through radical polymerization of styrene and itaconic acid, followed by esterification with epichlorohydrin. These bio-based polymers were mixed with commercial epoxy resin to create composites. The composites showed improved surface hardness, tensile strength and elasticity, with a 74.55–243% increase in elastic modulus compared to pure epoxy, especially for composites with styrene-based polymers. Wang et al., (2016) studied the epoxy thermoplastic blends. They reviewed the incorporation of thermoplastics into epoxy resins to improve mechanical properties, especially fracture toughness. The effects of thermoplastics like polysulfone, poly (ether sulfone), poly (ether imide) and poly (phthalazinone ether) on epoxy properties were discussed. Phase separation, toughening factors such as additive content, molecular weight and end groups were also discussed. Finally, the future research trends in thermoplastic/epoxy blends were highlighted. Huang et al., (2019) analyze the thermo-mechanical properties and morphology of epoxy resins with co-poly (phthalazinone ether nitrile). Their study examined the effect of adding PPEN, a thermoplastic, to an epoxy resin blend. Tests showed the blends were not uniform and had phase separation. The addition of 5% PPEN improved impact strength by 59% and slightly increased heat resistance (Tg). However, higher PPEN content reduced heat resistance. The blends became tougher while keeping good thermal properties, making them useful for improving epoxy resins. Kiattipornpithak et al. (2021) investigated the reaction mechanism and mechanical property improvement of poly (lactic acid) reactive blending with epoxy resin. Polylactic acid (PLA) was blended with epoxy resin to explore their effects on mechanical and thermal properties. Adding 0.5% epoxy increased PLA’s tensile strength from 57.5 MPa to 67 MPa, while 20% epoxy improved elongation at break to 12%. The PLA/epoxy blends showed smooth fracture surfaces and nanoparticle dispersion at higher epoxy content. The glass transition temperature decreased with more epoxy, but the Vicat softening temperature increased slightly. NMR confirmed the reaction between PLA’s – COOH groups and epoxy, improving the blends’ properties. This study aimed at further establishing the mechanical properties of epoxy blends. The objective of this research is to characterize the influence of the incorporation of PC in epoxy resins mechanical and thermal properties and to investigate the means and the mechanisms by which epoxy resins can be toughened effectively without significant loss in thermo-stability and other important properties.

The resin used was epoxy resin, which was modified with polycarbonate (PC, Lexan grade) and dissolved in 1, 2-Dichloromethane (CH₂Cl₂). Curing was achieved using di-amino-di-phenyl-methane (DDM) as the curing agent through an open-mold casting method. The curing process required temperatures of 150^οC and lasted for 60–90 min. Mixing was conducted mechanically with a constant stirrer. The physical properties of epoxy resin and polycarbonate are shown in Tables 1 and 2.

In solution blending, selected diluents are used to dissolve the component polymers, the diluents also lower the temperature and shear force necessary for satisfactory and uniform mixing without having any degradative effect on the bulk properties of the blend prepared. But removal of the diluents after solution blending may ultimately lead to prominent and uncertain changes in the phase morphology, thus weakening the blend and lowering its performance reliability. First the polycarbonate samples were cut from the sheet and put in the hot air oven for one hour at 100^οC to evaporate the moisture content of the small chips of the polycarbonate, if any. Then different percentage weights of PC samples viz. (2.5, 5, 7.5, 10 and 15%) are dissolved in DCM solvent, about 30 ml used for each case. Then leave them for about one overnight and even weight (%): 10 and 15% samples take time, about 2–3 days, to completely dissolve.

The PC solutions are mixed with epoxy batches, which are taken in 500 ml beakers (2.5 wt. % solution with 97.5% wt. of epoxy, 5 wt. % solution with 95 wt. % of epoxy, 7.5 wt. % solution with 92.5 wt. % of epoxy, 10 wt. % solution with 90 wt. % of epoxy and 15 wt. % solution with 85 wt. % of epoxy). Mixing takes place with the help of a constant mechanical stirrer. The mixing is done at 120^οC for all batches, and very easily the solvent gets evaporated. The mixing of the PC solution was done into the epoxy solution (hot) dropwise with too much care because there was a sudden exothermic reaction that occurred and the solvent got evaporated very fast. This process of mixing is carried out with constant string at 120–130^οC up to 5–6 h for each sample, and then after we get clear transparent solution of epoxy-polycarbonate blends. The curing agent DDM (∼27.50% weight percent of epoxy) was mixed with the PC-epoxy-1,138 blend at about 50–60^οC and a clear solution was obtained. The mixing of DDM was done in about 10 min.

The transparent clear solution obtained after mixing the curing agent (DDM, 28% by wt. of epoxy) with PC-epoxy blend was poured into an open mold of the dimensions (15  × 15 cm) at 120⁰C and kept at the same temperature for 1 h. Before pouring the blend solution into the open mold, a good-quality releasing agent was being put on the mold surface and mold was heated for half hours in the hot air oven. After completion of one hour, it again keeps on heating at 150⁰C for about 3–4 h for post-curing, i.e. optimum curing, and all reactive sites will undergo complete cross-linking. The blending compositions of epoxy and PC are shown in Table 3.

The tensile characteristics, like tensile strength and adhere to ASTM D-638 standards, evaluated through static tension tests. The specimen used is straight-sided with a consistent cross-section, featuring beveled tabs bonded at its ends. These properties hold significance for materials subjected to stretching or tension. The machine used to find out the tensile strength is the Universal Testing Machine. Impact properties of the polymeric materials are directly related to the overall toughness of the material. Toughness is defined as the ability of the polymeric material to absorb applied energy. The area under stress–strain curve is directly proportional to the toughness of a material. The impact properties of a material represent toughness, i.e. its capacity to absorb and dissipate energies under impact or shock loading. The Instrumented Impact Testing Machine is used for the Charpy impact test. The sample size is according to ASTM D256.

To assess the deterioration of blends and their combinations, thermogravimetric analysis was employed, analyzing powdered casting remnants. Conducted in a nitrogen environment using the Universal V4.2 E, TGA 2950 from TA Instruments USA, the experiment involved heating the samples at a rate of 10 ^οC per min. Graphs displaying residual weight versus temperature were generated as the samples were heated from room temperature to 1,000^οC. The instrument used for the TGA is TGA 2950. The heating is done at a heating rate of 10^οC min^-1 in a nitrogen atmosphere. The powder of the cured specimens, about 2–3 mg in weight, was taken for the TGA. The heating was done up to 800^οC.

Furthermore, the SEM approach was employed to study the morphology of the binary blends of Epoxy/PC blends as well as to study the morphology of their GRP composites. The fractured samples that were obtained after impact and flexural analysis were used for SEM analysis. The samples were scanned with a Carl Zeiss EVO-50*V_p Low-Vacuum Scanning Electron Microscope.

Figure 1 exhibited that the tensile strength of blends increases with PC contents and is maximum for the composition EPC3 (92.5:7.5) blend and also more a less same increment for composition EPC4 (90:10), although it is slightly less than the PC content of 7.5% composition. Because of this composition modifier, PC is going to start phase separate-out at minute extent and left as a plasticizer/crystallized impurities in the bulk. From the data table and with plot, it is clear that after 7.5 and 10% of PC compositions, the tensile strength going to sudden decreases even less than their unmodified composition value because of modifier (PC) self-coagulation and phase separation from epoxy matrix due to partial miscibility between both components. Thus, results showed that the tensile strength is maximum for the modified composition EPC3 (92.5:7.5) blend. The tensile strength corresponding to different blend compositions is depicted in Table 4.

Figure 2 concluded that the impact strength of blends gradually increases with modifier PC, up to the ratio of 7.5%. The fracture toughness for this composition increases about 1.8 times greater than the unmodified resin and 44% more than the base epoxy. Table 5, clearly depicted the impact strength corresponding to different blend ratios.

The thermogravimetric analysis (TGA) shown in Table 6, that the residual weight fluctuates with PC content. The residual weight is almost the same for the blend compositions at near 500^οC, and the highest residual weight is obtained for the PC 7.5% system due to the effective compatibilized blend. However, it has changed at nearly 800^οC and the maximum residual was obtained for the PC (5%) blend system at 641.79^οC meanwhile, for the PC content of 10% the value obtained was slightly lower than earlier at a higher temperature near 723.22^οC; consequently, it has higher stability toward temperature ranges as shown in Figures 3–8.

The SEM micrographs were taken of the fractured surface of the epoxy and their blends with PC. The different micrographs were taken for each specimen of the epoxy and their blends. From the SEM micrographs of the fractured surface of the casting, a characteristic single-phase homogeneous morphology following the direction of crack propagation was shown, which is clearly shown in Figure 9. It was observed that for the PC content of 10 and 15%, the SEM photograph presented as a rough fractured surface than EPC0, EPC1, EPC2 and EPC3, respectively.

Thus, some stress concentration developed at the interface of PC/epoxy blends. SEM micrograph PC (7.5%) explains the good compatibilized effective blend for EPC3 (92.5:7.5) composition. The SEM graphs show there is a very effective bonding/interaction at the interface of epoxy and PC.

The present study highlighted that the blends of epoxy with PC resin influence the thermal and mechanical properties of the cured system by making the modified resin better in properties up to 7.5% of PC content in the system than the parent one. Tensile strength of blends considerably increases up to compositions of 7.5 and 10%. Thus, it can be inferred that PC content of 7.5 and 10% systems have better compatibility. It also suggested that the PC content of 7.5 and 10% have nearly similar effects on several properties. Impact strength of blends gradually increases up to a composition of 7.5% PC in the epoxy resin. The fracture toughness for this composition increases about 1.8 times greater than the unmodified resin. TGA results showed that the thermal stability of the blends. The thermal stability is maximum for the blends with a composition of 7.5% and 10% PC content. The SEM micrograph explains the good compatibilized effective blend for the EPC3 (92.5:7.5) composition. The SEM photographs showed there is a very effective bonding/interaction at the interface of epoxy and PC with compositions of 7.5 and 10%, respectively. Hence, EPC3 and EPC4 compositions blend are more suitable for the various applications where significant thermo-mechanical properties are required. For the clear understanding of readers, the important findings are listed below:

• (1)

  Effect on mechanical properties:

  • •

    The addition of polycarbonate (PC) resin to epoxy enhances the thermal and mechanical properties of the cured system.

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    The epoxy–PC blends show a significant improvement in tensile strength, especially with 7.5 and 10% PC content.

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    Both 7.5 and 10% PC blends exhibit nearly identical positive effects on tensile strength, making them highly compatible in the system.

• (2)

  Impact strength:

  • •

    The impact strength of the blends gradually increases as the PC content rises, with the highest improvement seen at 7.5% PC.

  • •

    At 7.5% PC composition, the impact strength is notably better than the unmodified resin.

• (3)

  Fracture Toughness:

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    The fracture toughness of the epoxy–PC blend with 7.5% PC content is about 1.8 times greater than that of the unmodified epoxy resin.

  • •

    This significant improvement indicates better durability and resistance to cracking in the modified resin.

• (4)

  Thermal stability (TGA results):

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    Thermal gravimetric analysis (TGA) results show that the thermal stability of the blends improves with the addition of PC.

  • •

    The maximum thermal stability is observed in the blends with 7.5 and 10% PC content, indicating better heat resistance.

• (5)

  SEM analysis:

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    Scanning electron microscopy (SEM) micrographs reveal that the blend with 7.5% PC (EPC3) has a highly effective and well-compensated structure.

  • •

    The SEM images show a strong interaction at the interface between epoxy and PC in the 7.5 and 10% PC blends, confirming good compatibility and bonding.

• (6)

  Suitability for applications:

  • •

    The blends with 7.5 and 10% PC (EPC3 and EPC4 compositions) are particularly suitable for applications that require high thermo-mechanical properties.

  • •

    These blends show significant improvements in both thermal stability and mechanical strength, making them ideal for use in demanding conditions.