Mixed carbon nanomaterial/epoxy resin for electrically conductive adhesives

The increasing complexity of printed circuit boards (PCBs) due to miniaturization, increased the density of electronic components, and demanding thermal management during the assembly triggered the research of innovative solder pastes and electrically conductive adhesives (ECAs). Current commercial ECAs are typically based on epoxy matrices with a high load (>60%) of silver particles, generally in the form of microflakes. The present work reports the production of ECAs based on epoxy/carbon nanomaterials using carbon nanotubes (single and multi-walled) and exfoliated graphite, as well as hybrid compositions, within a range of concentrations. The composites were tested for morphology (dispersion of the conductive nanomaterials), electrical and thermal conductivity, rheological characteristics and deposition on a test PCB. Finally, the ECA’s shelf life was assessed by mixing all the components and conductive nanomaterials, and evaluating the cure of the resin before and after freezing for a time range up to nine months. The ECAs produced could be stored at −18 °C without affecting the cure reaction.This research was funded by the Portugal Incentive System for Research and Technological Development,Project in Co-Promotion n◦039479/2019 (Factory of the Future: Smart Manufacturing 2019–202

Evaluation of Hybrid Electrically Conductive Adhesives

An electrically conductive adhesive (ECA) is a composite material acting as a conductive paste, which consists of a thermoset loaded with conductive fillers (typically silver (Ag)). Many works that focus on this line of research were successful at making strides to improve its main weakness of low electrical conductivity. Most research focused on developing better silver fillers and co-fillers, or utilizing conductive polymers to improve its electrical conductivity, however, most of these works are carried out on small scale. In this work, we aim to produce larger quantities of hybrid ECA to successfully test its properties. Industry is interested in materials with superior physical properties. As such, rheological behavior and mechanical strength were explored as it has been theoretically hinted that incorporation of exfoliated graphene within the composite could impact those factors listed in a positive manner. In the first step of this project, pre-treated sodium dodecyl sulfate (SDS)-decorated graphene’s rheological properties were examined. An epoxy resin diglycidylether of bisphenol-A (DGEBA) was the main polymer used for this study: a well-known material that can behave either as a shear-thinning or shear-thickening material depending on the supplier. We showed how composites that contain graphene (Gr) had higher viscosities than ones that contained SDS decorated graphene Gr(s). Not only did we confirm that surfactant was a key factor in the decrease of viscosity, but we also report how Gr and Gr(s) had a special effect that suppresses the intrinsic shear thickening behavior of epoxy resin at weight concentrations (wt%) higher than 0.5 wt%. The results showed that Gr(s) is not only beneficial in terms of improving the conductivity of conventional ECAs, but it also acts as a solid lubricant that decreases the viscosity of the composite paste at higher weight concentrations. In the second step of the project, pre-treated SDS decorated graphene’s mechanical properties were examined. In specific, its lap-shear strength (LSS) as well as the effect of residual solvent when present in our hybrid ECA system were studied in order to follow up on the thermal results obtained from a previous study. We showed that our initial suspicion was correct as the LSS did decrease for all of the solvent-assisted formulations that contained Gr(s) ranging from 66 to 84%, however, we were not able to tell whether or not that decrease was caused by lower crosslinking density. Instead, we uncovered another reason for this decrease: bubble formation during the curing step. This suspicion was confirmed qualitatively through light microscopy and quantitatively through optical profilometry, where we present an increase in surface roughness for the solvent-assisted samples. Furthermore, by using SEM, we also confirmed that this bubble formation extends throughout the entire bulk material rather than just at the interface. Lastly, we investigated whether the use of solvent to assist in the mixing process significantly improves the electrical conductivity at a lower weight loading of Ag, and compared the electrical conductivity with that of the products prepared under the same higher weight loading of Ag using a solvent-free mixing method from previous work. Thirdly, we investigated another mechanical property of our hybrid ECAs through indentation tests, where we use Hertizan equations to characterize elastic modulus. Since we learned that the addition of Ag flakes is detrimental to the mechanical strength, we focused on the difference between the elastic moduli for Gr and Gr(s) in a solvent-free environment. In the last step of this project, we explored the use of a liquid-suspended co-filler (instead of carbon filler-based materials) in Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS): a conductive polymer that is frequently in conductive thin-films. We report that by using PEDOT:PSS as a conductive co-filler into the conventional ECA with 60 wt% of Ag, we observed higher conductivity equivalent to adding an extra 20 wt% of Ag into the system. Furthermore, we report that an increase of PEDOT:PSS in the composite appears to decrease the LSS of the material by 20%.

Rational design of electrically conductive polymer composites for electronic packaging

Electrically conductive polymer composites, i.e. polymers filled with conductive fillers, may display a broad range of electrical properties. A rational design of fillers, filler surface chemistry and filler loading can tune the electrical properties of the composites to meet the requirements of specific applications. In this dissertation, two studies were discussed. In the first study, highly conductive composites with electrical conductivity close to that of pure metals were developed as environmentally-friendly alternatives to tin/lead solder in electronic packaging. Conventional conductive composites with silver fillers have an electrical conductivity 1~2 orders of magnitude lower than that of pure, even at filler loadings as high as 80-90 wt.%. It is found that the low conductivity of the polymer composites mainly results from the thin layer of insulating lubricant on commercial silver flakes. In this work, by modifying the functional groups in polymer backbones, the lubricant layer on silver could be chemically reduced in-situ to generate silver nanoparticles. Furthermore, these nanoparticles could sinter to form metallurgical bonds during the curing of the polymer matrix. This resulted in a significant electrical conductivity enhancement up to 10 times, without sacrificing the processability of the composite or adding extraneous steps. This method was also applied to develop highly flexible/stretchable conductors as building block for flexible/stretchable electronics. In the second study, a moderately conductive carbon/polymer composite was developed for use in sensors to monitor the thermal aging of insulation components in nuclear power plants. During thermal aging, the polymer matrix of this composite shrank while the carbon fillers remained intact, leading to a slight increase in filler loading and a substantial decrease in the resistivity of the sensors. The resistivity change was used to correlate with the aging time and to predict the need for maintenance of the insulation component according to Arrhenius’ equation. This aging sensor realized real-time, non-destructive monitoring capability for the aging of the target insulation component for the first time.Ph.D

Electrically conductive self-healing polymer composite coatings

The goal of the research described herein is the fabrication and assessment of electrically conductive partially-cured epoxy coatings which, upon cracking, autonomously restore barrier, mechanical and electrical properties via a microcapsule based healing mechanism. Upon cracking, microcapsules in the crack path release the 'healing' solvent ethyl phenyl acetate (EPA), which locally swells the matrix, promoting crack closure and enabling the diffusion and subsequent reaction of the residual hardener in the vicinity of the crack. Two different self-healing coatings and two controls based on an electrically conductive epoxy resin with approximately 20% carbon nanotubes (CNTs) were fabricated. Electrochemical impedance spectroscopy was employed to evaluate the potential of the CNT and non-CNT containing encapsulated systems to restore relatively large cracks and thus restore the barrier function. An in situ electro-tensile test in a microscope revealed that electrical conductivity and mechanical properties were restored to 64% (23) and 81% ( 39) respectively, which correlated to crack closure. Under appropriate testing conditions the system showed successive damage-heal events in terms of electrical conductivity. (C) 2015 Elsevier B.V. All rights reserved

Preparation, characterization and performance evaluation of Nanocomposite SoyProtein/Carbon Nanotubes (Soy/CNTs) from Soy Protein Isolate

Formaldehyde-based adhesives have been reported to be detrimental to health. Petrochemical-based adhesives are non-renewable, limited and costly. Therefore, the improvement of environmental-friendly adhesive from natural agricultural products has awakened noteworthy attention. A novel adhesive for wood application was successfully prepared with enhanced shear strength and water resistance. The Fourier transmform infrared spectra showed the surface functionalities of the functionalized carbon nanotubes (FCNTs) and soy protein isolate nanocomposite adhesive. The attachment of carboxylic functional group on the surface of the carbon nanotubes (CNTs) after purification contributed to the effective dispersion of the CNTs in the nanocomposite adhesive. Hence, enhanced properties of FCNTs were successfully transferred into the SPI/CNTs nanocomposite adhesive. These unique functionalities on FCNTs however, improved the mechanical properties of the adhesive. The shear strength and water resistance of SPI/FCNTs was higher than that of the SPI/CNTs. SEM images showed the homogenous dispersion of CNTs in the SPI/CNTs nanocomposite adhesive. The carbon nanotubes were distributed uniformly in the soy protein adhesive with no noticeable clusters at relatively reduced fractions of CNTs as shown in the SEM images, which resulted into better adhesion on wood surface. Mechanical (shear) mixing and ultrasonication with 30 minutes of shear mixing both showed an improved dispersion of CNTs in the soy protein matrix. However, ultrasonication method of dispersion showed higher tensile shear strength and water resistance than in mechanical (shear) mixing method. Thermogravimetric analysis of the samples also showed that the CNTs incorporated increases the thermal stability of the nanocomposite adhesive at higher loading fraction. Incorporation of CNTs into soy protein isolate adhesive improved both the shear strength and water resistance of the adhesive prepared at a relatively reduced concentration of 0.3%.The result showed that tensile shear strength of SPI/FCNTs adhesive was 0.8 MPa and 7.25MPa at dry and wet state respectively, while SPI/CNTs adhesive had 6.91 MPa and 5.48MPa at dry and wet state respectively. There was over 100% increase in shear strength both at dry and wet state compared to the pure SPI adhesive. The 19% decrease in value of the new adhesive developed compared to the minimum value of ≥10MPa of European standard for interior wood application may be attributed to the presence of metallic particles remaining after purification of CNTs. The presence of metallic particles will prevent the proper penetration of the adhesive into the wood substrate. The type of wood used in this study as well as the processing parameters could also result into lower value compared to the value of European standard. Therefore, optimization of the processing parameter as well as the conversion of carboxylic acid group on the surface of the CNTs into acyl chloride group may be employed in future investigation. However, the preparation of new nanocomposite adhesive from soy protein isolate will replace the formaldehyde and petrochemical adhesive in the market and be of useful application in the wood industry.Civil and Chemical EngineeringM. Tech. (Chemical Engineering