Heat Dissipation in Epoxy/Amine-Based Gradient Composites with Alumina Particles: A Critical Evaluation of Thermal Conductivity Measurements

A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization

Photopolymerizations, in which the initiation of a chemical-physical reaction occurs by the exposure of photosensitive monomers to a high-intensity light source, have become a well-accepted technology for manufacturing polymers. Providing significant advantages over thermal-initiated polymerizations, including fast and controllable reaction rates, as well as spatial and temporal control over the formation of material, this technology has found a large variety of industrial applications. The reaction mechanisms and kinetics are quite complex as the system moves quickly from a liquid monomer mixture to a solid polymer. Therefore, the study of curing kinetics is of utmost importance for industrial applications, providing both the understanding of the process development and the improvement of the quality of parts manufactured via photopolymerization. Consequently, this review aims at presenting the materials and curing chemistry of such ultrafast crosslinking polymerization reactions as well as the research efforts on theoretical models to reproduce cure kinetics and mechanisms for free-radical and cationic photopolymerizations including diffusion-controlled phenomena and oxygen inhibition reactions in free-radical systems.

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.

Simultaneously enhanced dielectric properties and through-plane thermal conductivity of epoxy composites with alumina and boron nitride nanosheets

Dielectric materials with good thermal transport performance and desirable dielectric properties have significant potential to address the critical challenges of heat dissipation for microelectronic devices and power equipment under high electric field. This work reported the role of synergistic effect and interface on through-plane thermal conductivity and dielectric properties by intercalating the hybrid fillers of the alumina and boron nitride nanosheets (BNNs) into epoxy resin. For instance, epoxy composite with hybrid fillers at a relatively low loading shows an increase of around 3 times in through-plane thermal conductivity and maintains a close dielectric breakdown strength compared to pure epoxy. Meanwhile, the epoxy composite shows extremely low dielectric loss of 0.0024 at room temperature and 0.022 at 100 ℃ and 10-1 Hz. And covalent bonding and hydrogen-bond interaction models were presented for analyzing the thermal conductivity and dielectric properties.

Temperature-Triggered/Switchable Thermal Conductivity of Epoxy Resins

The pronouncedly low thermal conductivity of polymers in the range of 0.1–0.2 W m⁻¹ K⁻¹ is a limiting factor for their application as an insulating layer in microelectronics that exhibit continuously higher power-to-volume ratios. Two strategies can be applied to increase the thermal conductivity of polymers; that is, compounding with thermally conductive inorganic materials as well as blending with aromatic units arranged by the principle of π-π stacking. In this study, both strategies were investigated and compared on the example of epoxy-amine resins of bisphenol A diglycidyl ether (BADGE) and 1,2,7,8-diepoxyoctane (DEO), respectively. These two diepoxy compounds were cured with mixtures of the diamines isophorone diamine (IPDA) and o-dianisidine (DAN). The epoxy-amine resins were cured without filler and with 5 wt.-% of SiO₂ nanoparticles. Enhanced thermal conductivity in the range of 0.4 W·m⁻¹·K⁻¹ was observed exclusively in DEO-based polymer networks that were cured with DAN (and do not contain SiO₂ fillers). This observation is argued to originate from π-π stacking of the aromatic units of DAN enabled by the higher flexibility of the aliphatic carbon chain of DEO compared with that of BADGE. The enhanced thermal conductivity occurs only at temperatures above the glass-transition point and only if no inorganic fillers, which disrupt the π-π stacking of the aromatic groups, are present. In summary, it can be argued that the bisphenol-free epoxy-amine resin with an epoxy compound derivable from natural resources shows favorably higher thermal conductivity in comparison with the petrol-based bisphenol-based epoxy/amine resins.

Thermally enhanced polyolefin composites: fundamentals, progress, challenges, and prospects

The low thermal conductivity of polymers is a barrier to their use in applications requiring high thermal conductivity such as electronic packaging, heat exchangers, and thermal management devices. Polyolefins represent about 55% of global thermoplastic production, and therefore improving their thermal conductivity is essential for many applications. This review analyzes the advances in enhancing the thermal conductivity of polyolefin composites. First, the mechanisms of thermal transport in polyolefin composites and the key parameters that govern conductive heat transfer through the interface between the matrix and the filler are discussed. Then, the advantage and limitations of the current methods for measuring thermal conductivity are analyzed. Moreover, the progress in predicting the thermal conductivity of polymer composites using modeling and simulation is discussed. Furthermore, polyolefin composites and nanocomposites with different thermally conductive fillers are reviewed and analyzed. Finally, the key challenges and future directions for developing thermally enhanced polyolefin composites are outlined.

The Impact of the Seasoning Conditions on Mechanical Properties of Modified and Unmodified Epoxy Adhesive Compounds

The aim of this study was to analyse the impact of the adhesive samples seasoning conditions (temperature and time) on selected mechanical properties of four epoxy adhesive compounds (two unmodified and two modified ones). The samples were made of Epidian 53 epoxy resin mixed with the two different amine curing agents in appropriate stoichiometric proportions. A filler in the form of calcium carbonate (CaCO₃) powder was used as a modifier. The adhesive compound samples were cured for seven days. Six seasoning variants were used. Four of them were related with the seasoning time at ambient temperature of 24 ± 2 °C for: one month, two months, five months and eight months, respectively. Two other variants were related with seasoning at negative temperature (−10 ± 2 °C) for one month. The last variant (F) also included seasoning at ambient temperature (24 ± 2 °C) for five months right after seasoning in negative temperature. Cured and cylinder-shaped adhesive compound samples were subjected to compressive strength tests (according to the ISO 604 standard). The strength tests were performed using a Zwick/Roell Z150 testing machine. Based on the tests, it was observed that both temperature and time of seasoning influenced the adhesive’s mechanical properties. In the perspective of eight months, these changes were relatively minor for the samples seasoned at ambient temperature. The adhesive samples prepared for the tests were especially sensitive to negative temperature.