Thermosetting Resin System Research Articles

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.

Bio-based epoxy and unsaturated polyester resins: Research and market overview

The presented study provides an overview of the current research achievements and the emerging market of bio-based thermosetting polymers. Environmental attributes related to bio-based polymers trigger a steadily growing interest in this novel and promising field. Due to their importance among thermosets in terms of composite applications and quantity, this review focusses on epoxy and unsaturated polyester resins. Current studies are mainly concerned with alternative renewable raw materials to substitute fossil content and their synthesis to improve their end-properties. A common target is the increase of bio-based content within the cured resin. In spite of today’s efforts in research, the recent market review reveals only few commercially available bio-based thermosetting resin systems. However, they are commonly suited for a broad variety of processing methods and applications with bio-contents up to 75%.

Effect of atomic oxygen exposure on polybenzoxazine/POSS nanocomposites for space applications

A new thermoset resin system, based on a polybenzoxazine blend, has been subjected to high ATOX fluence (2.69 × 1021 atom/cm2), equating to a period of 300 days in low Earth orbit. Several baseline tests were carried out on the resin and the addition of POSS decreased ATOX erosion yield by 69 % compared with unmodified resin system. SEM and FTIR results confirm that the protection mechanism involves the formation of a silicon-rich surface layer in response to ATOX exposure, shielding the resin below from further erosion and principal components analysis was used to elucidate the degradation mechanism. Carbon fibre reinforced polymer (CFRP) laminates based on the new resin systems were tested for their mechanical properties. The addition of 6 wt% POSS leads to a 50 % increase in the energy required to initiate fracture and 41 % increase in the energy required to propagate a crack. Mode II fracture toughness is also improved by the addition of POSS (61.5 % increase in energy required to initiate a crack and 35.7 % increase in energy required to propagate it).

A Mechanical Performance Study of Dual Cured Thermoset Resin Systems 3D-Printed with Continuous Carbon Fiber Reinforcement.

Additive manufacturing (AM) is one of the fastest-growing manufacturing technologies in modern times. One of the major challenges in the application of 3D-printed polymeric objects is expanding the applications to structural components, as they are often limited by their mechanical and thermal properties. To enhance the mechanical properties of 3D-printed thermoset polymer objects, reinforcing the polymer with continuous carbon fiber (CF) tow is an expanding direction of research and development. A 3D printer was constructed capable of printing with a continuous CF-reinforced dual curable thermoset resin system. Mechanical performance of the 3D-printed composites varied with the utilization of different resin chemistries. Three different commercially available violet light curable resins were mixed with a thermal initiator to improve curing by overcoming the shadowing effect of violet light by the CF. The resulting specimens' compositions were analyzed, and then the specimens were mechanically characterized for comparison in tensile and flexural performance. The 3D-printed composites' compositions were correlated to the printing parameters and resin characteristics. Slight enhancements in tensile and flexural properties from some commercially available resins over others appeared to be the result of better wet-out and adhesion.

Recent progress on polyphenylene oxide-based thermoset systems for high-performance copper-clad laminates

With the rapid development of electronic information technology in the 5G era towards high integration, short propagation delay, and elevated assembly temperatures, more academic and industrial attention has been focused on high-frequency and high-speed copper-clad laminates (CCLs). Compared with conventional polymeric matrices, thermoset polyphenylene oxide (PPO) has become one of the most attractive resins applied in high-performance CCLs (HPCCLs) because of its excellent comprehensive properties, including outstanding dielectric properties, high thermal stability, great processibility, and low moisture absorption, etc. This review focuses on the history of the development of PPO prepolymers/oligomers, PPO-based thermoset resin systems, and PPO/inorganic filler composites to optimize the dielectric constant, dielectric loss, thermal conductivity, coefficient of thermal expansion, and mechanical properties. Moreover, some current challenges of PPO-based thermoset systems have been identified, such as developing feasible solutions to ensure the anti-aging properties for long-term reliability under harsh environments of high temperature, high humidity, and even high-frequency electromagnetic radiation. In general, more in-depth investigations of PPO-based thermoset systems for HPCCLs are required in the future.

“Hot Forming” Process vs Cycle Parametrization: Ancillary Materials, Risks by Contamination and Prepreg Aging

Aeronautical process globalization and the growth in number of composite materials suppliers have allowed the development of creativity since processes parametrization point of view as well as the use of ancillary materials that guarantee the product quality in the currently available resin systems in the composites world. Hot-forming process optimization in order to provide the development of high thickness parts and complex shape must meet a minimum parameters (temperature, time and vacuum) which are determinated by experimental methods and are in relation with: 1) Prepreg Material (tape / resin system); 2) Ancillary Materials, particulary the release film due to contamination risk of un-cured prepreg during hot-forming process; 3) Indication and control systems of temperature. Nowadays, the new thermoset resin systems are more complex due to an increase of the requirements from OEMs to manufacturers of composites in order to obtain a products optimization (functional and structural) in a more effective way.This aspect, joined to the study and the industrial capability analysis of a determinated equipment, allowed us the development of a tests campaign on structural parts of unidirectional prepreg (UD) to define the maximum and minimum temperature, time and ancillary materials to be used in a “hot-forming” process. Non contamination of the part and no material aging are guaranteed, as well as the internal quality after a process of consolidation by co-curing process in autoclave for any thermoset resin system available in the composite world.

Prepreg, carbon fiber, thermoset autoadhesive resin for sandwich structures: advantages, disadvantages and manufacturing challenges

It’s thoroughly known that sandwich structures in aeronautical sector are formed by a core material, which more usual format is honeycomb, between two facings or skins made from several prepreg material layers.  Nevertheless in order to obtain an adequate adherence between core and prepreg most of thermoset resin systems need the presence of a film adhesive placed among the core and prepreg to favour the adhesive menisci formation inside core cells. Main benefit asociate to autoadhesive prepregs usage in sandwich parts is adhesive elimination from part configuration which translates in a weight reduction of the structure. Present article is centered in autoadhesive prepreg materials 180º cure. After more than 3 years of work with several composite materials manufacturers and AIRBUS, key process parameters in an industrial environment have been defined along with basic characteristics in order to guarantee internal quality of the elements

Experimental and Numerical Comparison of Impact Behavior between Thermoplastic and Thermoset Composite for Wind Turbine Blades.

Damage generated due to low velocity impact in composite plates was evaluated focusing on the design and structural integrity of wind turbine blades. Impact properties of composite plates manufactured with thermoplastic and thermoset resins for different energy levels were measured and compared. Specimens were fabricated using VARTM (vacuum assisted resin transfer molding), using both matrix systems in conjunction with carbon, glass and carbon/glass hybrid fibers in the NCF (non-crimp fabric) architecture. Resin systems used were ELIUM 188O (thermoplastic) from Arkema Co., Ltd. and a standard epoxy reference, EPR-L20 from Hexion Co., Ltd. (thermoset). Auxiliary numerical finite element analyses were performed to better understand the tests physics. These models were then compared with the experimental results to verify their predictive capacity, given the intrinsic limitations due to their simplicity. Based in the presented results, it is possible to observe that ELIUM is capable to replace a conventional thermoset matrix. The thermoplastic panels presented similar results compared to its thermoset counterparts, with even a trend of less impact damage. Additionally, for both thermoplastic and thermoset resin systems, glass layups showed the lowest levels of damage while carbon panels presented the highest damage levels. Hybrid laminates can be applied as a compromise solution.

Fabrication technique using a core and cavity mold of a hybrid composite ballistic helmet

Material used for helmet technology has transformed moderately little since the introduction of aramids, which ballistic helmets still uses woven aramids with a thermoset resin system. Most widely used strengthening material for ballistic helmet is Kevlar fibers which comprise outstanding impact resistance and high strength to weight ratio. For Kevlar reinforcement and resin shaping process, composite materials uses molding process in the fabrication methods. The most vital fabrication method is the hand lay-up process. Hand lay-up process normally consists of laying a dry fabric layer by hand onto a mold to shape into desirable design and shape. After the lay-up process is done, resin is then applied to the dry plies. There are several accessible curing methods and the most commonly used method is to let the product to cure at room temperature. This paper presents the fabrication process of ballistic helmet using a hand lay-up fabrication process with a core and cavity mold. This paper also describes the helmet mold design, materials and processing methods.

Concept for Darcy-based viscosity measurement for fast-curing resin systems

In Resin Transfer Molding (RTM) a dry fibrous reinforcement is placed in a mold and a thermosetting resin system is then injected. Ever faster curing resins have been developed to minimize cycle times. For a thorough process design, the time- and temperature-dependent viscosity behavior needs to be known. However, the short curing times of sometimes less than 2 min often exceed the limitations of conventional measurement approaches. Necessary mixing, application and adjustment procedures lead to a “blind spot” especially at the particularly critical beginning of the reaction. This paper presents a specifically RTM-related concept for the viscosity characterization from mixing until gel point. It is based on generating a linear resin flow in a reference porous media and applying an algorithm based on Darcy's law. The overall approach and the algorithm are presented. The basic functionality is proven by using numerical simulation.

Mechanical Properties of Flax Tape-Reinforced Thermoset Composites.

Three thermoset resin systems—bio-epoxy, epoxy, and polyester-with 30 v% flax fiber reinforcement have been studied to identify the optimal fiber–resin combination in a typical composite structure. Tensile, interface and interlaminar shear strength together with flexural and impact damage tolerance were compared in this study. The results revealed that mechanical and interfacial properties were not significantly affected by the different resin systems. Microscopy studies reveal that epoxy laminates predominantly fail by fibre linear breakage, polyester laminates by fiber pull-out, and bio-epoxy laminates by a combination of the two. The higher failure strains and pull-out mechanism may explain the better impact damage tolerance of polyester composites. Flow experiments were also conducted, revealing faster impregnation and lower void content with polyester resin, followed by bio-epoxy, due to their lower viscosities. Overall, bio-epoxy resin demonstrates comparable performance to epoxy and polyester resins for use in (semi-)structural bio-composites.

Applicability of fiber Bragg grating sensors for cure monitoring in resin transfer molding processes

This article examines the use of fiber Bragg grating sensors for cure monitoring purposes in resin transfer molding processes. Within a resin transfer molding test series a thermoset epoxy-amine resin system was used in combination with a woven flax fiber reinforcement. Particular attention was paid on the location of the optical fiber sensor and its sensitive Bragg grating element inside the mold cavity. Three different installation approaches were tested and the correlation of the corresponding strain response with the actual cure state of the resin system was investigated at 50°C and 70°C isothermal cure temperature, respectively. We could demonstrate that characteristic, conspicuous strain changes are directly related to the sol–gel conversion of the thermoset polymer, which was analyzed considering different approaches for the gel-point detection based on rheological measurements. With the installation of the sensor inside a controllable, capsuled resin volume, we could achieve the most reliable strain response that provides capabilities to give in-situ information of the cure state beyond the gelation point.

Enhancement of Thermal and Mechanical Properties of Bismaleimide Using a Graphene Oxide Modified by Epoxy Silane.

A thermosetting resin system, based on bismaleimide (BMI), has been developed via copolymerization of 4,4′-diaminodiphenylsulfone with a newly synthesized graphene oxide modified using epoxy silane (ES-GO). The effect of ES-GO on the thermomechanical and mechanical properties of cured modified resin was studied. To evaluate the efficiency of the modified BMI systems, the composite samples using glass fiber cloth were molded and tested. Thermogravimetric analysis indicates that the cured sample systems displays a high char yield at lower concentrations of ES-GO (≤0.5 wt.%), suggesting an improved thermal stability. Using dynamic mechanical analysis, a marked increase in glass transition temperature (Tg) with increasing ES-GO content was observed. Analysis of mechanical properties reveals a possible effect of ES-GO as a toughener. The results also showed that the addition of 0.3 wt.% ES-GO maximizes the toughness of the modified resin systems, which was further confirmed by the result of analysis of fracture surfaces. At the same time, a molded composite with ES-GO showed improved mechanical properties and retention rate at 150 °C as compared to that made with neat resin.

Modelling and analysis of the thermal characteristic of thermoplastic composites from hybrid textiles during compression moulding

A promising strategy to decrease cycle times for manufacturing continuous-fibre–reinforced composites is processing of thermoplastic matrix systems due to their fast processability, since no cross-linking of molecular chains is required as for thermoset resin systems. Nevertheless, thermoplastic carbon fibre-reinforced plastics nowadays are predominantly manufactured with pre-impregnated sheet materials, which result in limited drapability and freedom of design. Hybrid textiles, consisting of thermoplastic and carbon fibres, can avoid these disadvantages. This class of reinforcements combines the drapability of dry textiles with thermoplastic matrices, which furthermore allow near net-shape processes. Relative shifting between the fibres and, consequently, draping is possible in a preforming step. The objective of this article is to expand our knowledge about hybrid textiles with regard to their thermal behaviour during compression moulding. Accordingly, the necessary parameters for modelling the thermal state of the dry textile and the impregnated laminate are investigated. Moreover, an in situ process analysis based on the reflection spectra of glass fibre-optical sensors, which are embedded inside the stacking, is investigated to provide information about the state of aggregation and to validate the thermal model.

Textile-Integrated Elastomer Surface for Fiber Reinforced Composites

Elastomer layers offer a wide range of surface functionalization options for fiber-reinforced polymer composites (FRPC), e.g. erosion protection or increased impact resistance. Goal of this study was to investigate if it is possible to prepare a textile-based semi-finished product with elastomeric surface, which can easily be used as outermost layer in different liquid composite molding (LCM) processes. For this purpose, different types of elastomer were pressed and vulcanized onto a biaxial glass fiber fabric. Target of this procedure was to reach partial immersion of the elastomer into the textile with remaining dry textile areas. The dry areas of the textile can later be impregnated with a thermoset resin system. The strategy is to have the transition region between elastomer and thermoset within one textile layer and to give a robust and easy to handle semi-finished-product in order to achieve a maximum bonding strength of the elastomer surface to the final composite part. It could be shown by micrographs and computer tomography that the elastomer only penetrates the textile at its boundary. A remarkable microimpregnation of individual filaments within the rovings does not take place. Concerning the manufacturing, since water evaporates during vulcanization, a sufficient process pressure must be maintained throughout the entire vulcanization process to ensure a pore-free elastomer. Peel-off tests similar to DIN EN 28510-1 on the finished composite showed a failure in the laminate and not in the boundary layer between laminate and elastomer, so that the desired high joint strength could be demonstrated.

Synthesis and characterization of PANI/P‐2M conductive composites: Thermal, rheological, mechanical, and electrical properties

AbstractIn this article, a new conductive thermosetting resin system was developed using Polyaniline with 2‐Methacryloyloxyethyl acid phosphate (P‐2 M) as a dopant. The resin has shown better steady state viscosity and enhanced mechanical properties as high as 3 ± 0.8 GPa as compared to our previously developed resin system of PANI‐DBSA. A DSC analysis was conducted to estimate the activation energy and the exact amount of energy for doping and curing process of the resin. The resin was synthesized with various wt% of PANI and each combination was characterized in detail. As the amount of PANI was increased, the electrical conductivity significantly enhanced. The maximum electrical conductivity obtained is 0.5 S/m, which is lower than other conductive resins but is many orders higher than epoxy resin. The uncured resin was studied for its viscosity change with shear rate and the constitution of PANI in the resin and we obtained a desirable steady‐state viscosity of the resin. Thus, the newly developed resin with efficient steady state viscosity in the favorable range, optimized electrical conductivity, and superior mechanical properties, can be applied into CFRP fabrication.

NIR LEDs and NIR lasers as feasible alternatives to replace oven processes for treatment of thermal-responsive coatings

Near-infrared (NIR) laser sources (800–1000 nm) can potentially reduce the processing time for curing by a fast heating and incorporation of NIR absorbers into the coating. The latter converts NIR laser light absorbed into thermal energy. This curing technique was successfully applied to one-component thermoset coatings based on blocked polyisocyanate/hydroxy-polyester and melamine formaldehyde/hydroxy-acrylate resins with heptamethine cyanines as near-infrared absorbing material. The laser curing was additionally compared with LED sources. In general, the curing time significantly decreases in comparison with traditional heat sources. Furthermore, the photopolymerization of acrylates or epoxides can be induced simultaneously by adding suitable initiators due to photochemical generation of radicals and cations. Curing of the thermoset resin system and the photopolymerization process created interpenetrating networks. Principally, the techniques reported based on photonic NIR sources may help to substitute oven techniques where thermal activation of curing reactions is typically induced by oven or heating with infrared radiators for coating applications.

Influence of thickness ratio and integrated weft yarn column numbers in shape memory alloys on the deformation behavior of adaptive fiber-reinforced plastics

The integration of functional material, such as actuator material, is an essential aspect for the multi-functionality and resource efficiency of fiber-reinforced plastics (FRPs). By integrating a textile-based actuator into reinforcing fabrics during the production process, single axis and intrinsic adaptive FRPs can be produced, and these can change their form according to requirements. However, this paper presents the effect of thickness ratio and integrated weft yarn column number in shape memory alloys on the deformation behavior of adaptive FRPs. In order to achieve this aim, shape memory alloys were converted into hybrid yarn in the form of a core-sheath structure before being textile-technologically integrated into the reinforcing fabrics using weaving technology. The thickness variation was achieved by the warp as well as weft yarn density and the type of interlacement. These preforms were infused by a thermosetting resin system. The adaptive FRPs were tested electro-mechanically with respect to their maximum deformation. Results reveal that the maximum deformation of adaptive FRPs is enhanced by increasing the thickness ratio and the integrated weft yarn column number in shape memory alloys.

Development and validation of recycled carbon fiber-based binder tapes for automated tape laying processes

The current growth in use of fiber reinforced polymer composites causes a strongly increasing amount of waste. Current approaches for fiber reinforced polymer composites recycling usually not exploit the potential of endless fibers as they are shortened during recycling and will not be properly aligned in the final product. Considering this, the present work aimed at the development of a recycling process for long recycled carbon fibers, where fiber length is preserved and load-related fiber orientation is possible. The starting point for the presented work was so-called slivers, which are long bundles of fibers resulting from a carding process that has been applied to fiber scrap. The main focus of this work was on the development of a binder mesh application rig that processes the sliver to a binder tape, processable in an automated tape laying process, which in turn required modifications to adapt to the novel tape. The functionality of the binder tape manufacturing process was validated with long recycled carbon fibers slivers with linear density of 4 g/m and fiber lengths between 70 and 120 mm. With the binder tape preform manufactured this way, two alternative routes for composite manufacturing were tested. First, the amount of binder was set so high that direct thermoplastic pressing of the preforms was possible. Second, the amount of binder was minimized, and the preforms were infiltrated with a thermoset resin system via resin transfer molding. While the thermoplastic route showed very deficient fiber–matrix adhesion, with the thermoset route, ≈68% of stiffness and ≈31% of strength of virgin fiber-based composites could be achieved in fiber direction in a unidirectional lay-up.

A Review of Recent Research on Bio-Based Epoxy Systems for Engineering Applications and Potentialities in the Aviation Sector

Epoxy resins are one of the most widely used thermosets in different engineering fields, due to their chemical resistance and thermo-mechanical properties. Recently, bio-based thermoset resin systems have attracted significant attention given their environmental benefits related to the wide variety of available natural resources, as well as the resulting reduction in the use of petroleum feedstocks. During the last two decades, considerable improvement on the properties of bio-sourced resins has been achieved to obtain performances comparable to petroleum-based systems. This paper reviews recent advances on new bio-based epoxy resins, derived from natural oils, natural polyphenols, saccharides, natural rubber and rosin. Particular focus has been given to novel chemical formulations and resulting mechanical properties of natural derived- epoxies, curing agents or entire systems, constituting an interesting alternative for a large variety of engineering applications, including the aviation sector. The present work is within the scope of the ECO-COMPASS project, where new bio-sourced epoxy matrixes for green composites are under investigation.