Rapid and Energy-Efficient Frontal Curing of Multifunctional Composites Using Integrated Nanostructured Heaters.

Current technologies for the manufacture of fiber-reinforced polymer composites are energy-intensive, environmentally unfriendly, and time-consuming and require expensive equipment and resources. In addition, composites typically lack key nonstructural functionalities (e.g., electrical conductivity for deicing, lightning strike protection, and structural health monitoring), which are crucial to many applications such as aerospace and wind energy. Here, we present a new approach for rapid and energy-efficient manufacturing of multifunctional composites without using traditional expensive autoclaves, ovens, or heated molds used for curing of composites. Our approach is predicated on embedding a thin conductive nanostructured paper in the composite layup to act as a resistive heater for triggering frontal polymerization of the matrix thermosetting resin of the composite laminate. Upon passing electric current, the nanostructured paper quickly heats up and initiates frontal polymerization, which then rapidly propagates through the thickness of the laminate, resulting in rapid curing of composites (within seconds to few minutes) irrespective of the size of the composite laminate. The integrated nanostructured paper remains advantageous during the service of the composite part by imparting new functionalities (e.g., deicing) to the cured composite, owing to its excellent electrical conductivity and electrothermal properties. In this work, we first study the influence of several composite processing parameters on the electrothermal properties of the nanostructured paper and determine the power required for rapid initiation of frontal polymerization. We then successfully fabricate a 10 cm × 10 cm composite panel within 1 min using only 4.49 kJ of energy, which is 4 orders of magnitude less than the energy consumed by the traditional bulk, oven-curing technique. Detailed experiments are conducted to provide an in-depth understanding of the effect of heater position, tooling material, and input power on frontal curing of composite laminates. The multifunctional response of produced composites is demonstrated by performing a deicing experiment, where a 50 × 50 × 3 mm3 cube of ice is completely melted within 3 min using an input power of 77 W.

3D Printing of Short-Carbon-Fiber-Reinforced Thermoset Polymer Composites via Frontal Polymerization.

3D printing of fiber-reinforced thermoset composites is desirable for rapid fabrication of 3D composite objects with minimal tooling. One of the main issues in 3D printing of thermoset composites is the low cure rates of matrix resins, which prevents rapid curing and rigidization of composite materials during the printing process and capturing the desired print geometry. Here, we demonstrate a new technique for in situ printing and curing of carbon-fiber-reinforced thermoset composites without any postcuring or postprocessing steps. Upon extrusion and deposition of the composite ink from a printing nozzle, the ink is cured via frontal polymerization, leading to rapid printing of high-quality composites. Tailoring the processing conditions allows for freeform or rapid, supported printing of 3D composite objects with zero void content and highly oriented carbon fiber reinforcements.

Electrothermal Performance of Heaters Based on Laser-Induced Graphene on Aramid Fabric.

Nanostructured heaters based on laser-induced graphene (LIG) are promising for heat generation and temperature control in a variety of applications due to their high efficiency as well as a fast, facile, and highly scalable fabrication process. While recent studies have shown that LIG can be written on a wide range of precursors, the reports on LIG-based heaters are mainly limited to polyimide film substrates. Here, we develop and characterize nanostructured heaters by direct writing of laser-induced graphene on nonuniform and structurally porous aramid woven fabric. The synthesis and writing of graphene on aramid fabric is conducted using a 10.6 µm CO2 laser. The quality of laser-induced graphene and electrical properties of the heater fabric is tuned by controlling the lasing process parameters. Produced heaters exhibit good electrothermal efficiency with steady-state temperatures up to 170 °C when subjected to an input power density of 1.5 W cm-2. In addition, the permeable texture of LIG-aramid fabric heaters allows for easy impregnation with thermosetting resins. We demonstrate the encapsulation of fabric heaters with two different types of thermosetting resins to develop both flexible and stiff composites. A flexible heater is produced by the impregnation of LIG-aramid fabric by silicone rubber. While the flexible composite heater exhibits inferior electrothermal performance compared to neat LIG-aramid fabric, it shows consistent electrothermal performance under various electrical and mechanical loading conditions. A multifunctional fiber-reinforced composite panel with integrated de-icing functionality is also manufactured using one ply of LIG-aramid fabric heater as part of the composite layup. The results of de-icing experiments show excellent de-icing capability, where a 5 mm thick piece of ice is completely melted away within 2 min using an input power of 12.8 W.

Proanthocyanidin encapsulation for sustained bioactivity in dentin bioadhesion: A two-year study.

OBJECTIVES: To determine the long-term effect on the stability of dentin-resin interfaces after the addition of polylactide (PLA) capsules containing proanthocyanidin (PAC) to adhesive resin. METHODS: Sub-micron (SM) and micron (M) size capsules containing PACs were produced using a combination of emulsification and solvent evaporation techniques and characterized. Human dentin surfaces (n = 8) were etched (35% glycolic acid) and primed (15% enriched Vitis vinifera extract solution - VVe), followed by the application of an experimental adhesive containing 0 (control), 1.5 wt% of SM or M PAC-filled PLA capsules light cured for 40 s. A crown was built using commercial composite. After 24 h-immersion (37 °C) in simulated body fluid, specimens were serially sectioned into resin-dentin beams. Microtensile bond strength (TBS), micro-permeability and fracture pattern were assessed immediately and after 1 and 2 years. Data were statistically analyzed using two-way ANOVA and post-hoc test (α = 0.05). RESULTS: Polydisperse capsules were manufactured with average diameter of 0.36 µm and 1.08 µm for SM and M, respectively. The addition of capsules did not affect TBS (p = 0.889). After 2 years, TBS significantly decreased in SM (p = 0.006), whereas M showed similar initial values (p = 0.291). Overall, less micro-permeability was found in M than the control and SM group (p < 0.001). After 2 years, fractured surfaces from capsule-containing groups failed within the adhesive layer while control fractured at the bottom of the hybrid layer. SIGNIFICANCE: The addition of PAC-filled PLA microcapsules in a dental adhesive did not affect the bond strength while increased and sustained the protection against micro-permeability in the interface, likely due to release of PACs.

Rapid synchronized fabrication of vascularized thermosets and composites.

Bioinspired vascular networks transport heat and mass in hydrogels, microfluidic devices, self-healing and self-cooling structures, filters, and flow batteries. Lengthy, multistep fabrication processes involving solvents, external heat, and vacuum hinder large-scale application of vascular networks in structural materials. Here, we report the rapid (seconds to minutes), scalable, and synchronized fabrication of vascular thermosets and fiber-reinforced composites under ambient conditions. The exothermic frontal polymerization (FP) of a liquid or gelled resin facilitates coordinated depolymerization of an embedded sacrificial template to create host structures with high-fidelity interconnected microchannels. The chemical energy released during matrix polymerization eliminates the need for a sustained external heat source and greatly reduces external energy consumption for processing. Programming the rate of depolymerization of the sacrificial thermoplastic to match the kinetics of FP has the potential to significantly expedite the fabrication of vascular structures with extended lifetimes, microreactors, and imaging phantoms for understanding capillary flow in biological systems.

Fabrication of pH-responsive monodisperse microcapsules using interfacial tension of immiscible phases.

Monodisperse, stimuli-responsive microcapsules are required for applications involving precise delivery of chemical payloads but are difficult to fabricate with high throughput and control over capsule geometry and shell wall properties, especially in the presence of organic solvents. In this paper, we adapt a facile technique based on the interfacial tension of immiscible phases for the generation of monodisperse emulsion templates and microcapsules. In this technique, either one (single emulsion) or two (double emulsion) dispersed phases are simultaneously delivered while reciprocating across the interface of a stationary immiscible continuous phase. The interfacial tension of the continuous phase results in the separation of a monodisperse droplet in every cycle. Monodisperse single emulsion-templated microcapsules based on cyclic poly(phthalaldehyde) (cPPA) and polymethacrylate (Eudragit E100) shell walls are formed with hydrophobic cores. The acid-triggered release of Eudragit and cPPA microcapsules containing an oil core is demonstrated in an acidic media. Tunable, monodisperse double emulsion templates with an aqueous core are formed with sizes ranging from 295 µm to 1200 µm and reciprocation frequencies of 1 Hz to 7 Hz. The double emulsion templates are converted to monodisperse, responsive microcapsules with a hydrophilic core through photocuring or selective solvent evaporation to form the polymer shell wall. Microcapsules with a variety of polymeric shell walls based on photocurable polyisocyanurate, cPPA and polylactide are fabricated. The acid-triggered release of cPPA microcapsules containing an aqueous core with a slower degradation rate is also demonstrated. We achieve excellent control over the emulsion templates and microcapsules, with polydispersity less than 2% and the ability to predict the size reliably based on process parameters. The cost-effectiveness, ease of fabrication and potential for scale-up make this technique very promising for fabrication of a diverse range of stimuli-responsive microcapsules.

Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization.

Thermoset polymers and composite materials are integral to today's aerospace, automotive, marine and energy industries and will be vital to the next generation of lightweight, energy-efficient structures in these enterprises, owing to their excellent specific stiffness and strength, thermal stability and chemical resistance1-5. The manufacture of high-performance thermoset components requires the monomer to be cured at high temperatures (around 180 °C) for several hours, under a combined external pressure and internal vacuum 6 . Curing is generally accomplished using large autoclaves or ovens that scale in size with the component. Hence this traditional curing approach is slow, requires a large amount of energy and involves substantial capital investment6,7. Frontal polymerization is a promising alternative curing strategy, in which a self-propagating exothermic reaction wave transforms liquid monomers to fully cured polymers. We report here the frontal polymerization of a high-performance thermoset polymer that allows the rapid fabrication of parts with microscale features, three-dimensional printed structures and carbon-fibre-reinforced polymer composites. Precise control of the polymerization kinetics at both ambient and elevated temperatures allows stable monomer solutions to transform into fully cured polymers within seconds, reducing energy requirements and cure times by several orders of magnitude compared with conventional oven or autoclave curing approaches. The resulting polymer and composite parts possess similar mechanical properties to those cured conventionally. This curing strategy greatly improves the efficiency of manufacturing of high-performance polymers and composites, and is widely applicable to many industries.

Low-Ceiling-Temperature Polymer Microcapsules with Hydrophobic Payloads via Rapid Emulsion-Solvent Evaporation.

We report a microencapsulation procedure based on rapid solvent evaporation to prepare microcapsules with hydrophobic core materials and low-ceiling-temperature polymer shell wall of cyclic poly(phthalaldehyde) (cPPA). We use and compare microfluidic and bulk emulsions. In both methods, rapid solvent evaporation following emulsification resulted in kinetically trapped core-shell microcapsules, whereas slow evaporation resulted in acorn morphology. Through the systematic variation of encapsulation parameters, we found that polymer-to-core weight ratios higher than 1 and polymer concentrations higher than 4.5 wt % in the oil phase were required to obtain a core-shell structure. This microencapsulation procedure enabled the fabrication of microcapsules with high core loading, controlled size, morphology, and stability. This procedure is versatile, allowing for the encapsulation of other hydrophobic core materials, i.e., mineral oil and organotin catalyst, or using an alternative low-ceiling-temperature polymer shell wall, poly(vinyl tert-butyl carbonate sulfone).

Encapsulation of grape seed extract in polylactide microcapsules for sustained bioactivity and time-dependent release in dental material applications.

OBJECTIVE: To sustain the bioactivity of proanthocyanidins-rich plant-derived extracts via encapsulation within biodegradable polymer microcapsules. METHODS: Polylactide microcapsules containing grape seed extract (GSE) were manufactured using a combination of double emulsion and solvent evaporation techniques. Microcapsule morphology, size distribution, and cross-section were examined via scanning electron microscopy. UV-vis measurements were carried out to evaluate the core loading and encapsulation efficiency of microcapsules. The bioactivity of extracts was evaluated after extraction from capsules via solvent partitioning one week or one year post-encapsulation process. Fifteen human molars were cut into 7mm×1.7mm×0.5mm thick mid-coronal dentin beams, demineralized, and treated with either encapsulated GSE, pristine GSE, or left untreated. The elastic modulus of dentin specimens was measured based on three-point bending experiments as an indirect assessment of the bioactivity of grape seed extracts. The effects of the encapsulation process and storage time on the bioactivity of extracts were analyzed. RESULTS: Polynuclear microcapsules with average diameter of 1.38µm and core loading of up to 38wt% were successfully manufactured. There were no statistically significant differences in the mean fold increase of elastic modulus values among the samples treated with encapsulated or pristine GSE (p=0.333), or the storage time (one week versus one year storage at room temperature, p=0.967). SIGNIFICANCE: Polynuclear microcapsules containing proanthocyanidins-rich plant-derived extracts were prepared. The bioactivity of extracts was preserved after microencapsulation.

Influence of the reaction stoichiometry on the mechanical and thermal properties of SWCNT-modified epoxy composites.

Previous studies suggest that carbon nanotubes (CNTs) have a considerable influence on the curing behavior and crosslink density of epoxy resins. This invariably has an important effect on different thermal and mechanical properties of the epoxy network. This work focuses on the important role of the epoxy/hardener mixing ratio on the mechanical and thermal properties of a high temperature aerospace-grade epoxy (MY0510 Araldite as an epoxy and 4,4'-diaminodiphenylsulfone as an aromatic hardener) modified with single-walled carbon nanotubes (SWCNTs). The effects of three different stoichiometries (stoichiometric and off-stoichiometric) on various mechanical and thermal properties (fracture toughness, tensile properties, glass transition temperature) of the epoxy resin and its SWCNT-modified composites were obtained. The results were also supported by Raman spectroscopy and scanning electron microscopy (SEM). For the neat resin, it was found that an epoxy/hardener molar ratio of 1:0.8 provides the best overall properties. In contrast, the pattern in property changes with the reaction stoichiometry was considerably different for composites reinforced with unfunctionalized SWCNTs and reduced SWCNTs. A comparison among composites suggests that a 1:1 molar ratio considerably outperforms the other two ratios examined in this work (1:0.8 and 1:1.1). This composition at 0.2 wt% SWCNT loading provides the highest overall mechanical properties by improving fracture toughness, ultimate tensile strength and ultimate tensile strain of the epoxy resin by 40%, 34%, 54%, respectively.

Quantitative dispersion analysis of inclusions in polymer composites.

The state of dispersion plays an important role on the performance of polymer nanocomposites. Dispersion is usually assessed based on the qualitative evaluation of microscopy micrographs. In this paper, a quantitative algorithm is introduced for analyzing the dispersion of inclusions in polymer composites using image analysis. In a binary image, on-pixels are considered as particle elements while off-pixels stand for matrix elements. To quantify the dispersion, the mean distance value between any matrix elements to their corresponding nearest neighboring particle element is measured. A dispersion index, DI, is then defined by comparing the image of interest with the associated uniformly dispersed case. Synthetic models were utilized to examine the sensitivity of the algorithm to various dispersity scenarios such as the effect of particle size, clustering, and cluster distribution. Optical micrographs of carbon nanotube modified epoxy with different states of dispersion were also employed to assess the applicability and functionality of the algorithm to real micrographs.

Dispersion stability in carbon nanotube modified polymers and its effect on the fracture toughness.

In this paper, the dispersion stability of multiwall carbon nanotubes (MWNTs) mixed with an epoxy resin is studied. An instrumented optical microscope with a hot stage was used to study the evolution of the carbon nanotubes (CNTs) dispersion during the cure of the resin. A new image processing approach is then introduced to quantify dispersion and identify the source of dispersion degradation during the cure. The results showed that the reduction of the resin viscosity at temperatures greater than 100 °C caused an irreversible re-agglomeration of the CNTs in the matrix. It was shown that the fine-tuning of the ratio and type of curing agent as well as the curing temperature directly affect the dispersion stability of MWNTs in the epoxy polymer. The dispersion quality was then directly correlated to the fracture toughness of the modified resin and a maximum of 20% improvement was achieved.