Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites

Work on dielectric spectroscopy of epoxy resin filled with nano-SiO2 at different relative humidities and temperatures is reported. Above the glass-transition temperature (Tg), dc-like imperfect charge transport (QDC or LFD) dominates the low frequency dielectric spectrum. Another mid-frequency relaxation process was found in the non-dried composites. Water also induces glass-transition temperature decreases, which can be measured both by dielectric spectroscopy and DSC. Both theory and experiment demonstrated that a higher water content could exist in nanocomposites than unfilled epoxy suggesting a bigger free volume when nanostructured. In our system, the hydrophilic surface of silica is likely to cause water to surround and lead to delamination of the epoxy from SiO2. This is a potential mechanical and dielectric weakness in the nanocomposites, which may lead to an ageing phenomenon. Hydrophobic surface group may reduce the water adsorption in nanocomposites

Electromagnetic wave absorption and structural properties of wide-band absorber made of graphene-printed glass-fibre composite

Lightweight composites combining electromagnetic wave absorption and excellent mechanical properties are required in spacecraft and aircraft. A one- dimensional metamaterial absorber consisting of a stack of glass fibre/epoxy layers and graphene nanoplatelets/epoxy films was proposed and fabricated through a facile air-spraying based printing technology and a liquid resin infusion method. The production process allows an optimum dispersion of graphene nanoplatelets, promoting adhesion and mechanical integration of the glass fibre/epoxy layers with the graphene nanoplatelets/epoxy films. According to experimental results, the proposed wide-band absorber provides a reflection coefficient lower than −10 dB in the range 8.5–16.7 GHz and an improvement of flexural modulus of more than 15%, with a total thickness of ∼1 mm. Outstanding electromagnetic wave absorption and mechanical performance make the proposed absorber more competitive in aeronautical and aerospace applications

The effect of water absorption on the dielectric properties of epoxy nanocomposites

In this research, the influence of water absorption on the dielectric properties of epoxy resin and epoxy micro-composites and nano-composites filled with silica has been studied. Nanocomposites were found to absorb significantly more water than unfilled epoxy. However, the microcomposite absorbed less water than unfilled epoxy: corresponding to reduced proportion of the epoxy in this composite. The glass transition temperatures of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The Tg decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20K as the humidity was increased from 0% to 100%. This implied that for all the samples, the amount of water in the resin component of the composites was almost identical. It was concluded that the extra water found in the nanocomposites was located around the surface of the nanoparticles. This was confirmed by measuring the water uptake, and the swelling and density change, as a function of humidity as water was absorbed. The water shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. This has been discussed in terms of a percolation model. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 5 monolayers. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive; this may extend for approximately 25 nm. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water surrounding a given size of nanoparticle appeared to be independent of the concentration of nanoparticles but approximately proportional to water uptake. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. Comments are made on the possible effect on electrical aging

Accelerated aging test on composite boat hulls produced by infusion process

The use of composite materials in the naval industry has been increasing especially over the last decades. The shipbuilding industry tends to use composite materials due to their low weight and good mechanical properties, typically in the construction of ship hulls. Since 2015, Arsenal do Alfeite, SA has been developing and building composite materials vessels, such as the construction of the Fiberglass Reinforced Polymer (FGRP) Coastal Patrol Boats. Thus, arise the need to understand the variation of the properties of the material during the lifespan and to study its degradation. In this study, the part of the hull considered the zone of interest was the one subject to the most severe conditions in the marine environment. Thus, the aim of this dissertation is to study the effect of the most critical factors on material degradation by comparing three distinct conditions: as currently produced, with and without paint coating, and the proposal for improvement through the addition of silica nanoparticles to the resin on the interface between composite material and PVC core. The most critical parameters for degradation were identified, namely ultraviolet radiation and humidity. The addition of silica nanoparticles to the resin aims to reduce the permeability of the interface layer and improve the material performance with respect to UV absorption in order to increase the material life. The set of specimens, produced by infusion, consisted on simulating the characteristics of the currently produced hull, the addition of 2 wt% nano-silica and the addition of nano-silica together with a layer of silicone paint. Monolithic specimens with original characteristics and nano-silica addition were also made. These specimens were tested in the accelerated aging chamber for 1000 hours. After the aging process, the degree of degradation of the specimens was identified and compared by visual and microscopic analysis of the areas of interest, and 3-point bending tests were performed to identify changes in the mechanical properties. A lower degradation level and improved mechanical properties were observed in the specimens with nano-silica addition. It was concluded that the addition of nano-silica increases the mechanical properties and marine life of glass fibre reinforced polymers which can be considered a competitive advantage in the marine industry

Hygrothermal Effect on Mechanical and Thermal Properties of Filament Wound Hybrid Composite

This study focused on the hygrothermal effect on filament wound glass-carbon/epoxy hybrid composite. Non-geodesic pattern of filament winding with a winding speed of 15.24–30.48 lm/min was used. Fiber tensioning weight of one kg and a winding angle of 30° were created to produce wound samples of the hybrid composite. The hybrid composite was wound by using a ±30° orientation with a total of six layers. Hygrothermal effect was conducted in a humidity chamber for three days (72 h). Control temperatures of 60 and 80°C were established, and humidity percentages of 50%, 70%, and 90% were used. Moisture absorption test showed that heat and humidity in most of the hybrid samples gradually increased. As a result, glass-carbon 80°C/90% showed the highest absorbed moisture at 0.77%. The involvement of highest heat and humidity showed the decline in the values of tensile and flexure strengths at 75.80 and 157.15 MPa, respectively. Fractography analysis using Stereo Microscope Stemi 2000-C indicated that glasscarbon/ epoxy 80°C/90% showed catastrophic damage, large crack, and longest delamination of fiber pullout at 10.39 mm. The fracture criterion revealed that the involvement of heat and humidity significantly affected the mechanical and physical properties of hybrid composite material

Environmental Study of Nano-Filler Embedded Fiber Reinforced Polymer Composite

Fiber reinforced polymer (FRP) composites have a huge demand for applications in diversified fields due of their unique combination of properties. Inspite of having many advantages, these composites are susceptible to degradation when exposed to harsh environmental conditions which limits their use. CNTs have proved to be an ideal and most suitable reinforcing element to manufacture advanced materials i.e nanocomposites. GFRP composites were fabricated with 0.1wt%, 0.3wt%, and 0.5wt% CNT. A low wt.% CNT up to 0.3% showed significant increase in mechanical properties such as flexural strength, ILSS, Storage modulus of GFRP due to better fiber/matrix interfacial bonding whereas a higher wt.% CNT showed not much improvement due to agglomeration of CNTs in the matrix. During the fabrication, period of service and storage of components made up of these polymer materials, they are operating under changing environments which affects the predictability and reliability of the long term as well as the short term properties and also their in-service performance. This work is also an attempt to highlight the degradations that may be caused to the GFRP as well as CNT reinforced GFRP composites on exposing to various environmental conditions such as UV radiation, 95% RH at 60C and Hot Water at 60C. Short beam shear test as well Dynamic Mechanical Analysis of conditioned samples in all the three cases showed a significant decrease in mechanical properties of the composite systems. There was also a very slight increase in the glass transition temperature due to these exposures. DMA study was conducted on all the composite systems with an isothermal holding at 90C and 110C with different holding time periods and their behaviour were analysed. Incorporation of a low wt.% CNT reinforcement is found to lessen the degradation effect of the exposure environment on the polymer matrix and 0.3wt% CNT-GE composite demonstrated an overall better performance under all kinds of exposure

Nanocellulose as building block for novel materials

This thesis describes the fabrication of novel green materials using nanocellulose as the building block. Bacterial cellulose (BC) was used as the nanocellulose predominantly in this work. BC is highly crystalline pure cellulose with an inherent fibre diameter in the nano-scale. A single BC nanofibre was found to possess a Young’s modulus of 114 GPa. All these properties are highly favourable for using BC as a nanofiller/reinforcement in green nanocomposite materials. In this work, the surface of BC was rendered hydrophobic by grafting organic acids with various aliphatic chain lengths. These surface-modified BC was used as nanofiller for poly(L-lactide) (PLLA). Direct wetting measurements showed that the BC nanofibre-PLLA interface was improved due to the hydrophobisation of BC with organic acids. This led to the production of BC reinforced PLLA nanocomposites with improved tensile properties. Nanocellulose can also be obtained by grinding of wood pulp, producing nanofibrillated cellulose (NFC). The surface and bulk properties of one type of NFC and BC were compared in this work. Furthermore, the reinforcing ability of NFC and BC was also studied and it was observed that there is no significant difference in the mechanical performance of NFC or BC reinforced nanocomposites. A novel method based on slurry dipping to coat sisal fibres with BC was developed to modify the surface of natural fibres. This method can produce either (i) a densely BC coating layer or (ii) “hairy” BC coated sisal fibres. Randomly oriented short BC coated sisal fibre reinforced hierarchical composites were manufactured. It was found that hierarchical (nano)composites containing BC coated sisal fibres and BC dispersed in the matrix were required to produce composites with improved mechanical properties. This slurry dipping method was also extended to produce robust short sisal fibre preforms. By infusing this preform with a bio-based thermosetting resin followed by curing, green composites with significantly improved mechanical properties were produced. BC was also used as stabiliser and nano-filler for the production of macroporous polymers made by frothing of acrylated epoxidised soybean oil followed by microwave curing

Hygrothermal damage mechanisms in graphite-epoxy composites

T300/5209 and T300/5208 graphite epoxy laminates were studied experimentally and analytically in order to: (1) determine the coupling between applied stress, internal residual stress, and moisture sorption kinetics; (2) examine the microscopic damage mechanisms due to hygrothermal cycling; (3) evaluate the effect of absorbed moisture and hygrothermal cycling on inplane shear response; (4) determine the permanent loss of interfacial bond strength after moisture absorption and drying; and (5) evaluate the three dimensional stress state in laminates under a combination of hygroscopic, thermal, and mechanical loads. Specimens were conditioned to equilibrium moisture content under steady exposure to 55% or 95% RH at 70 C or 93 C. Some specimens were tested subsequent to moisture conditioning and 100 cycles between -54 C and either 70 C or 93 C

Thermomechanical and hygroelastic properties of an epoxy system under humid and cold-warm cycling conditions

In this paper, we study the hygrothermal aging of an anhydride-cured epoxy under temperature and hygrometry conditions simulating those experienced by an aircraft in wet tropical or subtropical regions. Gravimetric and dimensional measurements were performed and they indicate that there are three stages in this aging process: the first one, corresponding to the early cycles can be called the “induction stage”. The second stage of about 1000 cycles duration, could be named the “swelling stage”, during which the volume increase is almost equal to the volume of the (liquid) water absorbed. Both the first and second stages are accompanied by modifications of the mechanical properties and the glass transition temperature. During the third (“equilibrium”) stage, up to 3000 cycles, there is no significant change in the physical properties despite the continuous increase of water uptake. This can be explained by the fact that only physically sorbed water can influence physical properties