Thermoconductive Thermosetting Composites Based on Boron Nitride Fillers and Thiol-Epoxy Matrices

Effect of Ambient Plasma Treatments on Thermal Conductivity and Fracture Toughness of Boron Nitride Nanosheets/Epoxy Nanocomposites

With the rapid growth in the miniaturization and integration of modern electronics, the dissipation of heat that would otherwise degrade the device efficiency and lifetime is a continuing challenge. In this respect, boron nitride nanosheets (BNNS) are of significant attraction as fillers for high thermal conductivity nanocomposites due to their high thermal stability, electrical insulation, and relatively high coefficient of thermal conductivity. Herein, the ambient plasma treatment of BNNS (PBNNS) for various treatment times is described for use as a reinforcement in epoxy nanocomposites. The PBNNS-loaded epoxy nanocomposites are successfully manufactured in order to investigate the thermal conductivity and fracture toughness. The results indicate that the PBNNS/epoxy nanocomposites subjected to 7 min plasma treatment exhibit the highest thermal conductivity and fracture toughness, with enhancements of 44 and 110%, respectively, compared to the neat nanocomposites. With these enhancements, the increases in surface free energy and wettability of the PBNNS/epoxy nanocomposites are shown to be attributable to the enhanced interfacial adhesion between the filler and matrix. It is demonstrated that the ambient plasma treatments enable the development of highly dispersed conductive networks in the PBNNS epoxy system.

Recent Advances in Limiting Fatigue Damage Accumulation Induced by Self-Heating in Polymer-Matrix Composites

The self-heating effect can be considered as a catastrophic phenomenon that occurs in polymers and polymer-matrix composites (PMCs) subjected to fatigue loading or vibrations. This phenomenon appears in the form of temperature growth in such structures due to their relatively low thermal conductivities. The appearance of thermal stress resulting from temperature growth and the coefficient of thermal expansion (CTE) mismatch between fibers and neighboring polymer matrix initiates and/or accelerates structural degradation and consequently provokes sudden fatigue failure in the structures. Therefore, it is of primary significance for a number of practical applications to first characterize the degradation mechanism at the nano-, micro- and macroscales caused by the self-heating phenomenon and then minimize it through the implementation of numerous approaches. One viable solution is to cool the surfaces of considered structures using various cooling scenarios, such as environmental and operational factors, linked with convection, contributing to enhancing heat removal through convection. Furthermore, if materials are appropriately selected regarding their thermomechanical properties involving thermal conductivity, structural degradation may be prevented or at least minimized. This article presents a benchmarking survey of the conducted research studies associated with the fatigue performance of cyclically loaded PMC structures and an analysis of possible solutions to avoid structural degradation caused by the self-heating effect.

D-GQDs Modified Epoxy Resin Enhances the Thermal Conductivity of AlN/Epoxy Resin Thermally Conductive Composites

This article proposes a method of increasing thermal conductivity (λ) by improving the λ value of a matrix and reducing the interfacial thermal resistance between such matrix and its thermally conductive fillers. D-GQDs (graphene quantum dots modified by polyetheramine D400) with a π-π-conjugated system in the center of their molecules, and polyether branched chains that are rich in amino groups at their edges, are designed and synthesized. AlN/DG-ER (AlN/D-GQDs-Epoxy resin) thermally conductive composites are obtained using AlN as a thermally conductive and insulating filler, using D-GQDs-modified epoxy resin as a matrix. All of the thermal conductivity, electrically insulating and physical-mechanical properties of AlN/DG-ER are investigated in detail. The results show that D-GQDs linked to an epoxy resin by chemical bonds can increase the value of λ of the epoxy-resin matrix and reduce the interfacial thermal resistance between AlN and DG-ER (D-GQDs-epoxy resin). The prepared AlN/DG-ER is shown to be a good thermally conductive and insulating packaging material.

Optimization of synthetic parameters of high-purity trifunctional mercaptoesters and their curing behavior for the thiol-epoxy click reaction

The direct esterification reaction between 3-mercaptopropionic acid (3-MPA) and trimethylolpropane (TMP) was conducted in the presence of various catalyst concentrations of p-toluenesulfonic acid (p-TSA) to examine the optimized synthetic conditions needed to produce high-purity trimethylolpropane-tris(3-mercaptopropionate) (TMPMP). The purity of the desired TMPMP and uncompleted side-product reduced as the acid catalyst concentration in this esterification reaction increased while the generation of thioester-based side-product increased. The equivalent ratio between epoxy and the manufactured TMPMP was maintained at 1 : 1 to monitor the curing behavior of the thiol–epoxy click reaction using the DSC technique. The thermal features of the base-catalyzed TMPMP-cured epoxy resin were assessed according to the purity of the TMPMP curing agent.

The direct esterification reaction between 3-mercaptopropionic acid and trimethylolpropane was conducted in the presence of various catalyst concentrations to find a synthetic route for high-purity trimethylolpropane-tris(3-mercaptopropionate).

Thermal Conductivity and Cure Kinetics of Epoxy-Boron Nitride Composites-A Review

Epoxy resin composites filled with thermally conductive but electrically insulating particles play an important role in the thermal management of modern electronic devices. Although many types of particles are used for this purpose, including oxides, carbides and nitrides, one of the most widely used fillers is boron nitride (BN). In this review we concentrate specifically on epoxy-BN composites for high thermal conductivity applications. First, the cure kinetics of epoxy composites in general, and of epoxy-BN composites in particular, are discussed separately in terms of the effects of the filler particles on cure parameters and the cured composite. Then, several fundamental aspects of epoxy-BN composites are discussed in terms of their effect on thermal conductivity. These aspects include the following: the filler content; the type of epoxy system used for the matrix; the morphology of the filler particles (platelets, agglomerates) and their size and concentration; the use of surface treatments of the filler particles or of coupling agents; and the composite preparation procedures, for example whether or not solvents are used for dispersion of the filler in the matrix. The dependence of thermal conductivity on filler content, obtained from over one hundred reports in the literature, is examined in detail, and an attempt is made to categorise the effects of the variables and to compare the results obtained by different procedures.

Delayed Thiol-Epoxy Photopolymerization: A General and Effective Strategy to Prepare Thick Composites

Photoinduced thiol-epoxy click polymerization possesses both the characteristics and advantages of photopolymerization and click reactions. However, the photopolymerization of pigmented or highly filled thiol-epoxy thick composites still remains a great challenge due to the light screening effect derived from the competitive absorption, reflection, and scattering of the pigments or functional fillers. In this article, we present a simple and versatile strategy to prepare thick composites via delayed thiol-epoxy photopolymerization. The irradiation of a small area with a light-emitting diode (LED) point light source at room temperature leads to the decomposition of a photobase generator and the released active basic species can uniformly disperse throughout the whole system, including unirradiated areas, via mechanical stirring. No polymerization was observed at room temperature and therefore the liquid formulations can be further processed with molds of arbitrary size and desired shapes. It is only by increasing the temperature that base-catalyzed thiol-epoxy polymerization occurs and controllable preparation of thick thiol-epoxy materials can be achieved. The formed networks display excellent uniformity in different radii and depths with comparable functionality conversions, similar T _g values, and thermal decomposition temperatures. The presented strategy can be applied to prepare thick composites with glass fibers possessing improved mechanical properties and dark composites containing 2 wt % carbon nanotubes.

The Use of Click-Type Reactions in the Preparation of Thermosets

Click chemistry has emerged as an effective polymerization method to obtain thermosets with enhanced properties for advanced applications. In this article, commonly used click reactions have been reviewed, highlighting their advantages in obtaining homogeneous polymer networks. The basic concepts necessary to understand network formation via click reactions, together with their main characteristics, are explained comprehensively. Some of the advanced applications of thermosets obtained by this methodology are also reviewed.

Thermal Conductive Composites Prepared by Addition of Several Ceramic Fillers to Thermally Cationic Curing Cycloaliphatic Epoxy Resins

Novel composite coatings prepared from 3,4-epoxy cyclohexylmethyl 3,4-epoxycyclohexane carboxylate (ECC) and different ceramic fillers have been prepared to improve the thermal dissipation of electronic devices. As latent cationic initiator, a benzylanilinium salt with triethanolamine has been used, which leads to a polyether matrix. Different proportions of Al₂O₃, AlN and SiC as fillers were added to the reactive formulation. The effect of the fillers selected and their proportions on the evolution of the curing was studied by calorimetry and rheometry. The thermal conductivity, thermal stability, thermal expansion coefficient and thermomechanical and mechanical properties of the composites were evaluated. An improvement of 820% in thermal conductivity in reference to the neat material was reached with a 75 wt % of AlN, whereas glass transition temperatures higher than 200 °C were determined in all the composites.

Stimuli-responsive thiol-epoxy networks with photo-switchable bulk and surface properties

In the present work, the versatile nature of o-nitrobenzyl chemistry is used to alter bulk and surface properties of thiol-epoxy networks. By introducing an irreversibly photocleavable chromophore into the click network, material properties such as wettability, solubility and crosslink density are switched locally by light of a defined wavelength. The synthesis of photo-responsive thiol-epoxy networks follows a photobase-catalyzed nucleophilic ring opening of epoxy monomers with photolabile o-nitrobenzyl ester (o-NBE) groups across multi-functional thiols. To ensure temporal control of the curing reaction, a photolatent base is employed releasing a strong amidine-type base upon light exposure, which acts as an efficient catalyst for the thiol epoxy addition reaction. The spectral sensitivity of the photolatent base is extended to the visible light region by adding a selected photosensitizer to the resin formulation. Thus, in the case of photoactivation of the crosslinking reaction the photorelease of the base does not interfere with the absorbance of the o-NBE groups. Once the network has been formed, the susceptibility of the o-NBE groups towards photocleavage reactions is used for a well-defined network degradation upon UV exposure. Sol-gel analysis evidences the formation of soluble species, which is exploited to inscribe positive tone micropatterns by photolithography. Along with the localized tuning of network structure, the irreversible photoreaction is exploited to change the surface wettability of thiol-epoxy networks. The contact angle of water significantly decreases upon UV exposure due to the photo-induced formation of hydrophilic cleavage products enabling the inscription of domains with different surface wettability by photolithography.

Effective Mechanical Properties and Thickness Determination of Boron Nitride Nanosheets Using Molecular Dynamics Simulation

Research in boron nitride nanosheets (BNNS) has evoked significant interest in the field of nano-electronics, nanoelectromechanical (NEMS) devices, and nanocomposites due to its excellent physical and chemical properties. Despite this, there has been no reliable data on the effective mechanical properties of BNNS, with the literature reporting a wide scatter of strength data for the same material. To address this challenge, this article presents a comprehensive analysis on the effect of vital factors which can result in variations of the effective mechanical properties of BNNS. Additionally, the article also presents the computation of the correct wall thickness of BNNS from elastic theory equations, which is an important descriptor for any research to determine the mechanical properties of BNNS. It was predicted that the correct thickness of BNNS should be 0.106 nm and the effective Young's modulus to be 2.75 TPa. It is anticipated that the findings from this study could provide valuable insights on the true mechanical properties of BNNS that could assist in the design and development of efficient BN-based NEMS devices, nanosensors, and nanocomposites.