Effects of Curing on Photosensitive Resins in SLA Additive Manufacturing

Different mechanical properties characterise the materials of 3D printed components, depending on the specific additive manufacturing (AM) process, its parameters, and the post-treatment adopted. Specifically, stereolithography (SLA) uses a photopolymerisation technique that creates solid components through selective solidification. In this study, 72 specimens were 3D printed using 12 commercial-grade methacrylate resins and tested under uniaxial tensile loads. The resin specimens were evaluated before and after curing. The recommended cure temperature and time were followed for all materials. The stress-strain curves measured during the testing campaign were evaluated in terms of maximum tensile strength, Young’s modulus, ductility, resilience, and toughness. The results reveal that the curing process increases the material stiffness and resistance to tensile loads. However, it was found that the curing process generally reduces the plasticity of the resins, causing a more or less marked brittle behaviour. This represents a potential limitation to the use of SLA 3D printing for structural elements which require some plasticity to avoid dangerous sudden failures.

Enhancement of Heat Ageing Properties of Epoxidised Natural Rubber Blend

In recent years, automotive hose and belt specifications have changed, requiring longer product life in terms of swelling, wear and heat ageing. Diene-based rubbers, such as natural rubber (NR) and styrene-butadiene rubber (SBR), have been widely used in diverse industries. However, some apparent defects such as limited ageing resistance and large compression set, have been demonstrated in some rubbers cured by sulfur or peroxides. In the making of general and industrial rubber goods, short production and sufficient scorch time is crucial especially by using an injection moulding. In this work, blend of Epoxidised Natural Rubber (ENR 25) and Butadiene was developed with two types of curing systems namely Conventional and Efficient Vulcanisation system. The aim of the study is to produce a satisfactory heat resistance rubber compounds and adequate process safety for rubber manufacturing. Results showed that curing system applied significantly affected thermal stability property of the compounds. Modulus and hardness of the blends appeared to decrease progressively with ageing. However, greater thermal stability especially ageing at 100°C for 200h was observed with compound containing efficient curing system compared to conventional curing system which corresponded to the cross link density attributed by the torque value and dynamic mechanical analysis. The results on stiffness however was effected by the curing system applied. The influence of cure temperature on the chemical crosslink density on both cure systems are being investigated. The network results will be correlated with the technical properties.

The Effect of Curing Temperature on the Morphology and Electro-Optical Properties of a Flexible UV-Cured Polymer Dispersed Liquid Crystal (PDLC)

In this work, the experimental data on flexible PDLC films at the industrial scale and the effect of cure temperature (Tc) on the morphology and electro-optical properties of a UV-cured PDLC system via polymer-induced phase separation (PIPS) method were studied. Under constant UV radiation intebsity and thickness, the morphological parameters such as dimension, number density, and volume fraction of the phase-separated liquid crystal droplets as well as the optical transmissions and switching voltages as a function of cure temperature were determined. The effect of cure temperature on the morphology and electro-optical properties of UV-cured PDLC films were analyzed.

Self-Healing in Mobility-Restricted Conditions Maintaining Mechanical Robustness: Furan–Maleimide Diels–Alder Cycloadditions in Polymer Networks for Ambient Applications

Two reversible polymer networks, based on Diels–Alder cycloadditions, are selected to discuss the opportunities of mobility-controlled self-healing in ambient conditions for which information is lacking in literature. The main methods for this study are (modulated temperature) differential scanning calorimetry, microcalorimetry, dynamic rheometry, dynamic mechanical analysis, and kinetic simulations. The reversible network 3M-3F630 is chosen to study the conceptual aspects of diffusion-controlled Diels–Alder reactions from 20 to 65 °C. Network formation by gelation is proven and above 30 °C gelled glasses are formed, while cure below 30 °C gives ungelled glasses. The slow progress of Diels–Alder reactions in mobility-restricted conditions is proven by the further increase of the system’s glass transition temperature by 24 °C beyond the cure temperature of 20 °C. These findings are employed in the reversible network 3M-F375PMA, which is UV-polymerized, starting from a Diels–Alder methacrylate pre-polymer. Self-healing of microcracks in diffusion-controlled conditions is demonstrated at 20 °C. De-gelation measurements show the structural integrity of both networks up to at least 150 °C. Moreover, mechanical robustness in 3M-F375PMA is maintained by the poly(methacrylate) chains to at least 120 °C. The self-healing capacity is simulated in an ambient temperature window between −40 and 85 °C, supporting its applicability as self-healing encapsulant in photovoltaics.

Dynamic Cure Kinetics and Physical-Mechanical Properties of PEG/Nanosilica/Epoxy Composites

This study investigated the effect of polyethylene glycol (PEG) and nanosilica (NS) on the physical-mechanical properties and cure kinetics of diglycidyl ether of bisphenol-A-based epoxy (DGEBA-based EP) resin. For this purpose, tensile and viscometry tests, dynamic mechanical thermal analysis (DMTA), and differential scanning calorimetry (DSC) were carried out under dynamic conditions. The results showed that adding NS and PEG enhances the maximum cure temperature as well as the heat of cure reaction (ΔH) in EP-NS, while it decreases in EP-PEG and EP-PEG-NS. The cure kinetic parameters of EP-PEG-NS were calculated by Kissinger, Ozawa, and KSA methods and compared with each other. The Ea calculated from the Kissinger method (96.82 kJ/mol) was found to be lower than that of the Ozawa method (98.69 kJ/mol). Also, according to the KAS method, the apparent Ea was approximately constant within the 10-90% conversion range. Tensile strength and modulus increased by adding NS, while tensile strength diminished slightly by adding PEG to EP-NS. The glass transition temperature (Tg) was calculated using DMTA which was increased and decreased by the addition of NS and PEG, respectively. The results of the viscometry test showed that the viscosity increased with the presence of both PEG and NS and it prevented the deposition of solid particles.

Modeling of Fatigue-Strength Development in Cold-Emulsion Asphalt Mixtures Using Maturity Method

Emulsion asphalts are cost-effective, environmentally friendly, and sustainable alternatives to hot-mix asphalts. Laboratory curing protocols currently used to simulate field curing of emulsion asphalts have been observed to offer conflicting predictions. This study employed the maturity method to account for the combined effects of temperature and time on fatigue-strength development in emulsion asphalts. An emulsion asphalt, composed of 55% reclaimed asphalt pavement, 45% virgin aggregates, 6.2% bitumen emulsion, and 4% pre-mix water was designed following the Asphalt Institute procedure. A total of 168 specimens from the mix were variously cured at 5 °C, 25 °C, 40 °C, and 50 °C for time intervals of 1, 3, 5, 7, 14, 21, and 28 days, before being tested for fatigue-strengths on the four-point bending test jig. It was observed that fatigue-strengths increased with an increase in cure temperature but decreased with an increase in cure duration. A parabolic hyperbolic fatigue-maturity model was developed from results of specimens cured at 5 °C, 25 °C, and 40 °C and validated with results from specimens cured at 50 °C. A strong correlation was observed between predicted fatigue-maturity and laboratory-determined fatigue-strengths at test strain levels between 125 µm/m and 200 µm/m. The study concluded that the laboratory characterization of emulsion asphalts should consider the curing history of the mix.

Low-temperature vulcanizable chloroprene rubber–bromobutyl rubber blend for encapsulation of undersea sensors: Nonconventional cure systems and reaction kinetics

Blends of chloroprene rubber (CR) and bromobutyl rubber (BIIR) are used in making the undersea sensors watertight by a process of encapsulation. The encapsulation process is conventionally done at high temperature approximately 150°C and above using high-temperature vulcanization (HTV). However, the new class of acoustic sensors like polyvinilidenefluride (PVDF) and thin film PZT are highly temperature sensitive and fragile in nature and hence they require low-temperature vulcanization (LTV) process to avoid damages and protect their full functionalities. However, conventional cure systems are not adoptable in LTV process and hence there is a need for the search of alternate cure systems. Not much work has been reported in this area. This article reports a nonconventional cure system vulcanizable with LTV and the associated reaction kinetics for a commonly used CR–BIIR blend for encapsulation of undersea sensors. Formulations have been attempted with cure systems based on red lead (Pb3O4) and zinc oxide (ZnO) for CR–BIIR blend in 80:20 weight ratio, instead of zinc oxide, magnesium oxide, and ethylene thiourea system, which are conventionally used in HTV. The cure parameters at low temperature between 70°C and 120°C and the activation energy for cure reactions ( E a) were estimated using MDR 2000 rheometer. Essential prerequisites like water resistance, electrical resistivity, and physicomechanical properties for sensor application are qualitatively analyzed for the blend cured at 90°C. The results reveal that the proposed nonconventional cure systems are able to bring down the cure temperature of CR–BIIR blend to 90°C from 150°C enabling the suitability of the materials for undersea sensor encapsulation.

Adamantane-Modified Graphene Oxide for Cyanate Ester Resin Composites with Improved Properties

The conjugation of graphene and polymers has attracted great attention for the fabrication of functional hybrid nanomaterials. Here, we demonstrate the modification of graphene oxide (GO) with adamantane (AMT) through the diimide-activated amidation reaction. The modification of GO with AMT improves the dispersion and decreases the interfacial polarization of GO, causing a lower dielectric constant for the fabricated GO/AMT hybrid materials. The structures of GO/AMT were studied by Fourier transform infrared spectroscopy and Raman spectroscopy. Furthermore, the mechanical properties, thermal stability, and dielectric constant of GO/AMT composites were measured at a low cured temperature using various techniques, such as differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical thermal analysis. It was found that the synthesized GO/AMT materials with different contents were blended into cyanate ester (CE) resins, resulting in a lower cure temperature, smaller dielectric constant, higher thermal stability, and stronger water resistance. It is expected that this novel GO/AMT-CE material will have potential applications for replacing traditional thermosetting resins.