Properties Of Thermosets Research Articles

LINEAR DENSITY CHRONICLES: INVESTIGATING THE IMPACT OF E-GLASS THERMOSET AND THERMOPLASTIC COMPOSITES

This comprehensive investigation delves into the mechanical characteristics of E-glass reinforcement at varying linear densities in two-dimensional (2D) woven fabric-reinforced composites employing both thermoplastic and thermoset matrices. By scrutinizing tensile strength, flexural strength, edge-wise impact resistance and out-of-plane impact properties, the study optimizes composite materials and sheds light on the influence of linear density on the mechanical properties of thermoset and thermoplastic composites. Key insights underscore the superior in-plane load-bearing capacity of thermoset composites under quasi-static conditions, contrasting with the exceptional edge-wise and out-of-plane impact resistance exhibited by thermoplastic composites. Furthermore, the study reveals that thermoset composites outperform their thermoplastic counterparts in tensile and flexural properties, with discernible deviations in quasi-static mechanical properties with increasing linear density. In both thermoplastic and thermoset composites, specimens that had lower linear density reinforcement demonstrated enhanced mechanical performance under quasi-static circumstances. Nevertheless, when subjected to dynamic conditions, thermoplastic composites exhibited this pattern, whereas thermoset composites demonstrated divergent characteristics. In the context of low-velocity impact events, it was shown that Thermoplastic 600 Tex Glass Fabric Reinforced Composite (TP6G2DFRC) exhibited greater performance compared to all other specimens, even those with higher linear density. Conversely, in thermoset composites, Thermoplastic 1200 Tex Glass Fabric Reinforced Composite (TS12G2DFRC) demonstrated notable superiority over Thermoplastic 600 Tex Glass Fabric Reinforced Composite (TS6G2DFRC), despite possessing a higher linear density

Curing reactions, reaction kinetics, and latency of epoxy resin cured with L‐tryptophan and L‐tyrosine

AbstractIn the research on amino acids as bio‐based curing agents for epoxy resins, L‐tryptophan and L‐tyrosine have emerged as promising alternatives. Understanding the curing reactions and reaction kinetics is crucial for designing an appropriate curing regime to tailor the mechanical properties of thermosets. This study provides a comprehensive analysis of the curing reactions, curing kinetics, and latency of epoxy resin cured with L‐tryptophan or L‐tyrosine in the presence of urea‐based accelerators. L‐tryptophan involves three distinct curing reactions, corresponding to its three functional groups, while data on L‐tyrosine as a curing agent is less definitive. The degree of cure can be reliably predicted using model‐free kinetics for all resin systems studied. Overall, the inclusion of accelerators facilitates rapid curing at elevated temperatures while maintaining adequate processability for up to four weeks of storage at room temperature.

Influence of Different Filler Systems on the Thermal Conductivity and Mechanical Properties of Thermosets.

The changing demands in terms of the compactness and performance of electronic devices increase the role of thermal management within this application field. Because polymers and especially thermosets have a low thermal conductivity, filler systems have to be used to improve their performance and make thermosets suitable for applications. So far, several factors influencing thermal conductivity have been defined; however, the combined investigation of mechanical properties as another important value in terms of applications has not been realized. Therefore, this paper analyzes thermal conductivity based on the orientation of fillers and a contact path as well as mechanical properties in different material systems. In addition to the matrix material, the filler type and grade and the geometry and size were varied. It was shown that boron nitride provides the best values in terms of thermal conductivity after orientation along the flow path is realized. An aluminum silicate was defined as the best filler system in terms of mechanical properties. However, the boron nitride was found to reveal the greatest potential in terms of applications, since the mechanical properties are likely to be increased by the usage of finishing additives such as silane.

Vinyl methyl oxazolidinone and dimethyl itaconate as styrene substitutes for conventional high‐temperature unsaturated polyester resins

AbstractStyrene substitution is a major area of interest in the unsaturated polyester resins (UPR) industry and research. For most styrene‐free UPRs the unsaturated polyester (UP) formulation was tailored for less toxic reactive diluent monomers (RDM). The main challenge is finding a RDM that can directly substitute styrene in industrial UPR formulations without deterioration of resin and thermoset properties. Therefore, this study concerns vinyl methyl oxazolidinone (VMOX®) as a reactive diluent monomer for industrial high‐temperature UPRs. VMOX® and styrene were compared with dimethyl itaconate (DMI), methyl methacrylate, 2‐hydroxyethyl methacrylate, isobornyl methacrylate, triethylene glycol dimethacrylate and 1,4‐butanediol dimethacrylate. Further, the properties of produced resins and thermosets were investigated. The most striking result was comparable glass transition temperature, crosslink density, and residual monomer content of VMOX®‐ and styrene‐based thermosets. Subsequently, DMI was applied as a reactive co‐diluent for VMOX®‐based resins and thermosets. Significant improvement of VMOX® conversion during resin curing was observed. In conclusion, VMOX® is an excellent candidate for direct styrene substitution in industrial high‐temperature UPRs. Furthermore, the application of DMI as a reactive co‐diluent improves the reactivity of VMOX®‐based resins. The application of VMOX® and reactive co‐diluent monomer certainly creates new possibilities for direct styrene substitution in conventional UPR formulations.

Toughening and Imparting Deconstructability to 3D-Printed Glassy Thermosets with "Transferinker" Additives.

Thermoset toughness and deconstructability are often opposing features; simultaneously improving both without sacrificing other mechanical properties (e.g., stiffness and tensile strength) is difficult, but, if achieved, could enhance the usage lifetime and end-of-life options for these materials. Here, a strategy that addresses this challenge in the context of photopolymer resins commonly used for 3D printing of glassy, acrylic thermosets is introduced. It is shown that incorporating bis-acrylate "transferinkers," which are cross-linkers capable of undergoing degenerative chain transfer and new strand growth, as additives (5-25 mol%) into homemade or commercially available photopolymer resins leads to photopolymer thermosets with substantially improved tensile toughness and triggered chemical deconstructability with minimal impacts on Young's moduli, tensile strengths, and glass transition temperatures. These properties result from a transferinker-driven topological transition in network structure from the densely cross-linked long, heterogeneous primary strands of traditional photopolymer networks to more uniform, star-like networks with few dangling ends; the latter structure more effectively bear stress yet is also more easily depercolated via solvolysis. Thus, transferinkers represent a simple and effective strategy for improving the mechanical properties of photopolymer thermosets and providing a mechanism for their triggered deconstructability.

An Investigation into Mechanical Properties of 3D Printed Thermoplastic-Thermoset Mixed-Matrix Composites: Synergistic Effects of Thermoplastic Skeletal Lattice Geometries and Thermoset Properties.

This study aims to develop thermoplastic (TP) and thermoset (TS) based mixed matrix composite using design dependent physical compatibility. Using thermoplastic-based (PLA) skeletal lattices with diverse patterns (gyroid and grid) and different infill densities (10% and 20%) followed by infiltration of two different thermoset resin systems (epoxy and polyurethane-based) using a customized FDM 3D printer equipped with a resin dispensing unit, the optimised design and TP-TS material combination was established for best mechanical performance. Under uniaxial tensile stress, the failure modes of TP gyroid structures with polyurethane-based composites included 'fiber pull-out', interfacial debonding and fiber breakage, while epoxy based mixed matrix composites with all design variants demonstrated brittle failure. Higher elongation (higher area under curve) was observed in 20% infilled gyroid patterned composite with polyurethane matrix indicating the capability of operation in mechanical shock absorption application. Electron microscopy-based fractography analysis revealed that thermoset matrix properties governed the fracture modes for the thermoplastic phase. This work focused on the strategic optimisation of both toughness and stiffness of mixed matrix composite components for rapid fabrication of construction materials.

Understanding the structure-property relationship of anhydride-cured epoxidized vegetable oils: Modeling and molecular dynamics simulation

The development of high-performance thermosetting resins derived from epoxidized vegetable oil (EVO) has received significant attention in terms of the development of low-carbon economies. In this work, the all-atomic cross-linked models of anhydride-cured EVO were constructed and five cross-linking pathways involving distinct cure reactions were considered in the modelling process. The evolution of the number of distinct cure reactions and reactive groups of the cross-linked network under each pathway were monitored. The effects of the type of cure reaction, cure reaction probability, epoxy functionality of monomer, molar ratio of anhydride to epoxy, and anhydride structure on mass density, gelation transition, bulk properties, thermal properties, and mechanical properties of EVO-based thermosets were investigated in detail. The results show that the network exhibited minor difference in cure reactions but apparent thermo-mechanical properties of the thermosets. Etherification of epoxy groups and dehydration condensation caused the unfavorable thermo-mechanical properties. The glass transition temperature (Tg) is most affected by the molar ratio of anhydride to epoxy; whereas the Young's modulus is sensitive to the epoxy functionality of monomer. It is expected that the current work provides deep insights into the network formation and performance development of anhydride-cured EVO thermosets.

Enhancing thixotropic properties of epoxy resin and mechanical properties of epoxy resin thermosets by polyethyleneimine functionalized carbon nanotubes

Epoxy resin (ER) thermosets are widely applied in various fields due to their high mechanical strength, thermal stability and chemical corrosion resistance. However, the brittleness of ER thermosets and low thixotropic property of ER matrix largely limits their further development. Here, this article develops branched polyethyleneimine (PEI) functionalized carbon nanotube (PEI@CNT) as high-performance nanofiller for simultaneously strengthening and toughening ER thermosets. Benefiting from good dispersity and rich amine groups of PEI@CNT, effective interfacial bonding between PEI@CNT and ER is achieved. The ER thermoset with 0.5 wt% PEI@CNT shows increase in tensile strength by 24.8 % and fracture elongation by 30.0 % compared with pristine ER thermoset. The enhanced mechanical properties of PEI@CNT modified ER thermosets can be ascribed to the chemical interaction between PEI@CNT and ER matrix as well as the formed crack propagation, which can dissipate a lot of fracture energy, further improving the strength and toughness of the ER composites. More importantly, the thixotropic property of ER matrix is largely improved after adding the PEI@CNT filler. This work reports a high-performance nanofiller for enhancing mechanical properties of ER thermosets and processability of ER matrix.

Effects of Chemical Composition and Cross-Linking Degree on the Thermo-Mechanical Properties of Bio-Based Thermosetting Resins: A Molecular Dynamics Simulation Study.

Bio-based epoxy resins have received significant attention in terms of concerns regarding carbon emission. Epoxidized soybean oil (ESO) derived from sustainable feedstock has been widely used to blend with traditional diglycidyl ether of bisphenol-A (DGEBA) to replace some of the petroleum-based components. In this work, molecular dynamics (MD) simulations were applied to track the network formation and predict the performance of methyl hexahydrophthalic anhydride (MHHPA)-cured ESO/DGEBA blend systems. The effects of ESO content and cross-linking degree on the mass density, volumetric shrinkage, glass transition temperature (Tg), coefficient of thermal expansion (CTE), Young's modulus, yield strength, and Poisson's ratio of the epoxy resin were systematically investigated. The results show that systems with high ESO content achieve gelation at low cross-linking degree. The Tg value, Young's modulus, and yield strength increase with the increase in cross-linking degree, but the CTE at the glassy state and Poisson's ratio decrease. The comparison results between the simulated and experimental data demonstrated that the MD simulations can accurately predict the thermal and mechanical properties of ESO-based thermosets. This study gains insight into the variation in thermo-mechanical properties of anhydride-cured ESO/DGEBA-based epoxy resins during the cross-linking process and provides a rational strategy for optimizing bio-based epoxy resins.

Mechanical Behaviour of As-Manufactured and Repaired Aligned Discontinuous Basalt Fibre-Reinforced Vitrimer Composites.

The aim of this research is to investigate basalt as a natural mineral-based fibre together with a vitrimeric resin as a sustainable alternative to standard composite materials. Vitrimers combine the properties of thermoset and thermoplastic polymers, enabling the repair of specimens and hence prolonging the lifetime of the composite material. The micro-mechanical characteristics between the basalt fibres and the vitrimer resin are reported and shown to match those of a standard Skyflex K51 epoxy resin. Discontinuous (4 mm) basalt fibres were employed to produce aligned discontinuous fibre-reinforced composites (ADFRCs) using the high-performance discontinuous fibre (HiPerDiF) technology. The mechanical characteristics of the laminates were investigated through tensile testing and the fracture zones were analysed under a scanning electron microscope. By normalising the results by their respective fibre volume fraction, it was discovered that the vitrimer-basalt ADFRCs exhibited, on average, a 4% higher strength and a 25% higher stiffness compared to their basalt epoxy counterparts. The repair potential of the vitrimer ADFRC specimens was explored during low-temperature compression repair. Two approaches were tested using double-sided local- and full-patch repair. Both successfully recovered a significant amount of their prime strength. In conclusion, the potential of the sustainable vitrimer-basalt composite is shown by its competitive mechanical performance. Combining this with the manufacturing flexibility, repair potential, and recyclability of the material, the vitrimer-basalt composite seems to be a competitive alternative to standard glass epoxies.

Bio-Based Thermosetting Resins: From Molecular Engineering to Intrinsically Multifunctional Customization.

Recent years have witnessed a growing interest in bio-based thermosetting resins in terms of environmental concerns and the desire for sustainable industrial practices. Beyond sustainability, utilizing the structural diversity of renewable feedstock to craft bio-based thermosets with customized functionalities is very worthy of expectation. There exist many bio-based compounds with inherently unique chemical structures and functions, some of which are even difficult to synthesize artificially. Over the past decade, great efforts are devoted to discovering/designing functional properties of bio-based thermosets, and notable progress have been made in antibacterial, antifouling, flame retardancy, serving as carbon precursors, and stimuli responsiveness, among others, largely expanding their application potential and future prospects. In this review, recent advances in the field of functional bio-based thermosets are presented, with a particular focus on molecular structures and design strategies for discovering functional properties. Examples are highlighted wherein functionalities are facilitated by the inherent structures of bio-based feedstock. Perspectives on issues regarding further advances in this field are proposed at the end.

Resolving the relaxation complexity of vitrimers: Time-temperature superpositions of a time-temperature non-equivalent system

Vitrimers are polymer networks that, thanks to covalent bond exchange, combine desirable properties of thermoplastic and thermosets, such as flowability and insolubility. For this reason, vitrimers are considered to be good candidates for a number of innovative applications from self-healing soft robots to hard reprocessable materials. All these applications are related to the unusual thermomechanical behavior of vitrimers, consequence of the non-trivial interplay between the polymer network dynamics and the thermally activated chemical link exchange. Here we use solid-state rheology to investigate the properties of a recently developed epoxy-based vitrimer. The rheological analysis demonstrates that the mechanical spectrum is composed of two relaxation processes with distinct activation energies that are associated with glass dynamics and covalent bond exchange, respectively. This makes the material thermo-rheologically complex and time temperature equivalence does not apply. Nonetheless, thanks to mechanical spectral analysis in a wide range of stiffness, time and temperature, we are able to depict the time-temperature-relaxation landscape in an enough precise way to account for the two dynamical processes and recombine them to predict the mechanical moduli in a wide interval of frequencies (21 decades), from low temperatures (close to room temperature) to high temperatures (above the glass transition temperature).

Vinyl methyl oxazolidinone as reactive diluent to overcome the solubility limitations of highly polar unsaturated polyesters

AbstractVinyl methyl oxazolidinone (VMOX®) is studied as a new reactive diluent (RD) for highly polar unsaturated polyesters (UPs). Molecules with special structure properties are extensively investigated as components for unsaturated polyester resins (UPRs) and thermosets, particularly for high‐temperature applications. Special diols with a cyclic structure like isosorbide and tricyclodecanedimethanol or special diols with side groups like 2‐methyl‐2‐propyl‐1,3‐propanediol and 2‐ethyl‐2‐butyl‐1,3‐propanediol can provide interesting properties. Bio‐based diols such as isosorbide and 1,3‐butanediol improve the sustainability profile. However, UPs based on such special diols have a higher polarity, limiting their solubility in styrene. This demands a RD, which provides adequate solubility and is suitable for high‐temperature applications. Therefore, our study compares the solubility of highly polar UPs based on special diols in styrene and VMOX®. UPs with different formulations, polarities, and Hansen parameters were synthesized and characterized. The solubility sphere of styrene was determined experimentally with the synthesized UPs. The influence of the special diols on the thermoset properties was investigated in dynamic mechanical analysis measurements. In conclusion, VMOX® as a RD enables full utilization of special diols properties due to the wide solubility window. Moreover, a new class of high‐temperature styrene‐free UP resin was developed.

Coupling effect of ionic liquids on the mechanical, thermal, and chemical properties of polymer composites: A succinct review

AbstractThe growing study of polymer composites has motivated scientists to explore more methodologies to enhance the final demanding properties of polymer composites. Polymer composites have commonly been fabricated using thermosets or thermoplastics with synthetic or natural fillers. Lately, ionic liquids have been applied as coupling agents for polymer composites. In this succinct review, two types of ionic liquids, specifically imidazolium‐ and phosphonium‐based ionic liquids that are applied as coupling agents, are identified. Additionally, the synthesis of ionic liquids with different precursors and reactions is explained. The coupling effect of ionic liquids on the mechanical, thermal, and chemical properties of polymer composites is also succinctly reviewed. The knowledge shown in this review gives a more transparent comprehension of the fabrication of polymer composites coupled with ionic liquids and the essential properties of polymer composites. In conclusion, most imidazolium‐ and phosphonium‐based ionic liquids can improve several mechanical, thermal, and chemical properties of thermoset and thermoplastic composites.

Thermo-responsive properties of self-healable thermosets based on epoxidized soybean oil

In this work, epoxidized soybean oil (ESO) based thermosetting materials cured with citric acid (CA) and modified with low molecular weight 4′-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) nematic liquid crystal were synthesised and characterized. In order to find adequate curing conditions, the non-isothermal differential scanning calorimetry (DSC) was performed and the formation of thermosetting systems was confirmed by Fourier transform infrared spectroscopy (FTIR). As expected, the addition of HOBC nematic liquid crystal affected strongly the curing reaction of ESO/CA mixture. Thus, HOBC liquid crystal phase was partially miscible with ESO for low HOBC content, while its phase separation from ESO/CA matrix was detected for more than 20 wt% HOBC content. Thermal behaviour and thermal stability depend strongly on HOBC content leading, for high HOBC content, to thermosetting systems with nematic-isotropic transition characteristic for HOBC liquid crystal. Optical properties of HOBC-ESO/CA thermosetting systems confirmed the capability of HOBC liquid crystal phase to switch reversibly photoluminescence properties applying temperature. Moreover, an improvement of the thermo-responsive behaviour (shorter switching time and sharper switching temperature range) can be obtained modifying developed system with a small amount of poly(ε-caprolactone-b-lactide) (CLLA) block copolymer. Designed HOBC-ESO/CA thermosetting systems maintained self-healing properties attributed to ESO/CA thermosetting system. Thus, HOBC-ESO/CA thermosetting system modified with CLLA block copolymer was able to reach an interface width reduction of 98% after 2 h of self-healing process at 160 °C. This research work allowed to develop thermosetting systems, which combine self-healing properties of ESO/CA matrix with thermo-responsive properties of HOBC liquid crystal phase. Designed thermosetting materials can broad potential applications of ESO/CA thermosetting system.

Towards sustainable reprocessable structural composites: Benzoxazines as biobased matrices for natural fibers

In this work, we synthesized and investigated three fully biobased benzoxazine matrices containing exchangeable ester bonds for natural fiber composites. The thermoset properties were investigated and the transesterification behavior was assessed. The obtained polymers show high tunability. Using isosorbide as the starting building block, the thermoset exhibits a glass transition of 130 °C, a tensile modulus of 2.5 GPa, and thermal stability leading to degradation occurring after 270 °C with 31% char at 800 °C. All formulations stress relax under catalyst-free conditions within minutes with properties recovery superior to 80%. Finally, flax composites were manufactured. We highlight strong affinities between the matrices and the fibers through high mechanical performances with a modulus over 30 GPa and stress at break of 400 MPa in the longitudinal direction. 5 GPa modulus and 47 MPa stress at break were found in the transverse direction. Excellent fire retardancy properties, with self-extinguishment and UL-94 V1 classification were obtained for the isosorbide-based/flax composite. The obtained composites were able to be welded with comparable results to glued ones, paving the way to processable laminates and stable cured prepreg perfectly suited for transportation-engineered applications.

Moderated ionic bonding for water-free recyclable polyelectrolyte complex materials.

While nature extensively uses electrostatic bonding between oppositely charged polymers to assemble and stabilize materials, harnessing these interactions in synthetic systems has been challenging. Synthetic materials cross-linked with a high density of ionic bonds, such as polyelectrolyte complexes, only function properly when their charge interactions are attenuated in the presence of ample amounts of water; dehydrating these materials creates such strong Coulombic bonding that they become brittle, non-thermoplastic, and virtually impossible to process. We present a strategy to intrinsically moderate the electrostatic bond strengths in apolar polymeric solids by the covalent grafting of attenuator spacers to the charge carrying moieties. This produces a class of polyelectrolyte materials that have a charge density of 100%, are processable and malleable without requiring water, are highly solvent- and water-resistant, and are fully recyclable. These materials, which we coin "compleximers," marry the properties of thermoplastics and thermosets using tailored ionic bonding alone.

Robustly Curing Epoxy Novolacs by Linear and Cyclic Siloxane-Functionalized Eugenol: Impact of Topologies of Cross-Linkers on Properties of Thermosets

Robustly Curing Epoxy Novolacs by Linear and Cyclic Siloxane-Functionalized Eugenol: Impact of Topologies of Cross-Linkers on Properties of Thermosets

Role of additive size in the segmental dynamics and mechanical properties of cross-linked polymers.

Thermoset materials often involve the addition of molecular and nanoparticle additives to alter various chemo-physical properties of importance in their ultimate applications. The resulting compositional heterogeneities can lead to either enhancement or degradation of thermoset properties, depending on the additive chemical structure and concentration. We tentatively explore this complex physical phenomenon through the consideration of a model polymeric additive to our coarse-grained (CG) thermoset investigated in previous works by simply varying the size of additive segments compared to those of polymer melt. We find that the additive modified thermoset material becomes chemically heterogeneous from additive aggregation when the additive segments become much smaller than those of the thermoset molecules, and a clear evidence is observed in the spatial distribution of local molecular stiffness estimated from Debye-Waller factor 〈u2〉. Despite the non-monotonic variation trends observed in dynamical and mechanical properties with decreasing additive segmental size, both the structural relaxation time and moduli (i.e., shear modulus and bulk modulus) exhibit scaling laws with 〈u2〉. The present work highlights the complex role of additive size played in the dynamical and mechanical properties of thermoset polymers, which should provide a better understanding for the glass formation process of cross-linked polymer composites.

Gallic acid ester-based flexible UV-shielding epoxy thermosets with tunable properties

Epoxy thermosets are usually limited by their inherent brittleness and low-impact resistance. Henceforth, a series of bio-based epoxy resins were synthesized from gallic acid (GA) esters. Varying the chain length of flexible poly(ethylene glycol) (PEG) enabled tailoring the thermomechanical properties of the thermosets. Decreasing the chain length of the epoxy prepolymer, resulted in the increase of tensile strength from 2.89 to 15.18 MPa with a decrease in elongation from 121.7 to 16.91% and an increase in viscosity of the epoxy resins from 5.25 to 228.46 Pa.s. The low viscosity of epoxy resins with longer chain lengths offers good processability and broadens their relevance as reactive diluents for resins of high viscosity. Additionally, the thermosets exhibited decent transparency and UV shielding ability with satisfactory biodegradable attributes. The study revealed a direct correlation between chain length and thermomechanical properties of the epoxy thermosets.