Enhanced mechanical properties of epoxy composites using cellulose micro- and nano-crystals

Epoxy polymers are commonly utilized in structural applications due to their high bearing capacity and excellent chemical resistance. However, their inherent brittleness poses a significant challenge for their use in high shock and fracture strength products. To address this shortcoming, fillers can be incorporated into the polymer during preparation. In this study, we aimed to investigate the effect of incorporating cellulose-based fillers, namely cellulose nanocrystals (CNCs) and microcrystalline cellulose (MCC), on the mechanical properties of epoxy polymer composites. The study evaluated the impact of various factors, including filler concentration, particle size, and moisture content, on the mechanical properties of the composites. The results demonstrated that the incorporation of CNC or MCC powders at concentrations below 5% could enhance the mechanical properties of the resulting epoxy composites without adversely affecting their surface and thermal properties. The maximum tensile strength and fracture toughness of the filler-based epoxy composites were achieved at 2 and 4 wt% for CNCs and MCC, respectively. CNCs with a smaller particle size distribution were found to be much more effective than MCC in improving the mechanical properties of the epoxy composites. Furthermore, utilizing dried fillers resulted in a higher improvement in tensile strength, which was achieved at lower filler concentrations.     

Mechanical and thermal properties of waterborne epoxy composites containing cellulose nanocrystals

Cellulose nanocrystals (CNCs) are reinforcing fillers of emerging interest for polymers due to their high modulus and potential for sustainable production. In this study, CNC-based composites with a waterborne epoxy resin matrix were prepared and characterized to determine morphology, water content, and thermal and mechanical properties. While some CNC aggregation was observed, the glass transition temperature (T_g) and modulus for the composites increased with increasing CNC content. Relative to neat epoxy, at 15 wt.% CNC the storage modulus increased by 100%, the T_g increased from 66.5 °C to 75.5 °C, and tensile strength increased from 40 MPa to 60 MPa, suggesting good adhesion between epoxy and CNC surfaces exposed to the matrix. Additionally, no additional water content resulting from CNC addition were observed. These results provide evidence that CNCs can improve thermomechanical performance of waterborne epoxy polymers and that they are promising as reinforcing fillers in structural materials and coatings.     

Investigation of CNT modification of epoxy resin in CFRP strengthening systems

The use of carbon fiber‐reinforced polymers (CFRPs) to reinforce old structures has become popular in recent years. In this study, the chemical structure of the epoxy resin used as the bonding agent in the CFRP strengthening system was modified by dispersing multi‐walled carbon nanotubes (MWCNTs) in order to improve the performance of the strengthening system. Composites were fabricated with different mixing orders employing the solvent‐assisted dispersion method and ultrasonic mixing. Thermogravimetric analysis, dynamic mechanical analysis, and tensile tests were conducted to investigate the effect of CNT dispersion and fabrication method on the thermal and mechanical properties of epoxy composite. In addition, the temperature‐dependent tensile behavior of fabricated composites was studied by performing tensile tests at elevated temperatures. The morphology of CNT/epoxy composites was characterized using scanning electron microscopy (SEM). Fourier transform infrared (FTIR) was also used to show the influence of solvent on the molecular structure of composites. Based on the experimental results, the decomposition temperature of the epoxy resin was heightened by 15°C as a result of solvent‐assisted dispersion of nanotubes. However, the glass transition temperature (T_g) was slightly reduced due to the solvent effect. FTIR analysis revealed that the solvent negatively affects the curing process of epoxy composite. A considerable enhancement was recorded in the tensile properties as a result of CNT infusion. This was attributed to the homogeneous dispersion of nanotubes which was shown by SEM images. Using solvent to disperse nanotubes led to the reduction of tensile strength of the epoxy composite at elevated temperature due to the lower T_g. POLYM. COMPOS. 37:1021–1033, 2016. © 2014 Society of Plastics Engineers     

Effect of processing method on cellulose nanocrystal/polyethylene-co-vinyl alcohol composites

Cellulose nanocrystals (CNCs) have emerged as fillers of interest to the polymer nanocomposite community due to their inherent properties and renewable precursors. However, challenges persist to incorporate CNCs into polymer matrices due to component incompatibility and/or thermal stability limitations of the nanoparticles. Therefore, the objective of this research is to examine the efficacy of different processing methods in producing CNC/polymer composites. In this work, CNCs were incorporated into polyethylene-co-vinyl alcohol (EVOH) using either a solution casting method or a multi-step method involving the same solution casting method followed by a melt mixing step. The resulting neat EVOH and composite materials were characterized to understand how the viscoelastic character and mechanical properties were influenced by CNC loading and processing method. The results of characterization experiments and micromechanical modeling suggested that the nanoparticle networks produced by each method were different and that a combined solution-melt processing method is beneficial in producing composites with improved properties, particularly at higher CNC loadings. This processing strategy may be more broadly applied to other nanocomposites.     

Structure-property relationships of controlled epoxy networks with quantified levels of excess epoxy etherification

Epoxy thermosets are commonly formulated with an excess of epoxy resin to ensure complete reaction of co-reactive hardeners and to optimize performance. However, the degree to which the reaction of the excess epoxy resin contributes to the thermal and mechanical properties of the thermoset is incompletely understood. In this report, the preparation of controlled epoxy thermosets containing varying amounts of excess epoxy resin having essentially complete excess epoxy conversion is described. The extent of conversion was determined using a solid-state ¹³C NMR method with enhanced resolution due to solvent swelling of the thermosets. This etherification reaction increases the crosslink density of the epoxy thermosets but uniquely affects the thermal and mechanical properties of the materials. Significant property differences observed with respect to analogous thermosets made by varying crosslink density using different extender/hardener ratios are the sensitivity of T_g to the crosslink density and enhanced fracture toughness and tensile yielding with reduced bulk density.     

Development and characterization of bionanocomposites based on poly(3‐hydroxybutyrate) and cellulose nanocrystals for packaging applications

Poly(3‐hydroxybutyrate) (PHB)‐based bionanocomposites were prepared using various percentages of cellulose nanocrystals (CNCs) by a solution casting method. CNCs were prepared from microcrystalline cellulose using sulfuric acid hydrolysis. The influence of CNCs on PHB properties was evaluated using differential scanning calorimetry, Fourier transform infrared spectroscopy, X‐ray diffraction, thermogravimetry and tensile testing. Vapor permeation and light transmission of the materials were also measured. Differential scanning calorimetric tests demonstrated that CNCs were effective PHB nucleation agents. Tensile strength and Young's modulus of PHB increased with increasing CNC concentration. Moreover, the PHB/CNC bionanocomposites exhibited reduced water vapor permeation compared to neat PHB and had better UV barrier properties than commodity polymers such as polypropylene. It was found that nanocomposites with 6 wt% of CNCs had the optimum balance among thermal, mechanical and barrier properties. © 2016 Society of Chemical Industry     

Comparison of the role of milled glass and carbon fibers on mechanical properties of (bisphenol A)‐based epoxy composites

The main target of the current work was to study the mechanical properties of milled E‐glass, S‐glass, and high‐strength (carbon fiber)‐reinforced epoxy composites. At first, tensile behavior of the as‐received fibers was evaluated by conducting different tensile tests. Afterwards, the effects of employing an integral blended coupling agent on the performance of the pure epoxy were investigated by microhardness tests and optical microscopic images. Then, the epoxy composites were prepared simply by mixing and stirring 1, 3, and 5 wt% of the milled fibers with the epoxy resin and its hardener. The effects of mixture degassing and addition of the coupling agent to the mixture were examined based on the mechanical properties of the fabricated composites. Also, scanning electron microscope macro‐ and micrographs of the transverse and longitudinal fracture surfaces were used to study the fracture behavior and identify the active toughening mechanisms. The best results were obtained for the degassed and modified milled (carbon fiber epoxy)‐reinforced composite, which enhanced the tensile strength, elongation, Young's modulus, and toughness up to 12%, 17%, 19%, and 27%, respectively. The current study shows that the composite not only is cost effective but also offers better mechanical properties. J. VINYL ADDIT. TECHNOL., 24:130–138, 2018. © 2016 Society of Plastics Engineers     

Poly(methyl methacrylate)‐grafted cellulose nanocrystals: One‐step synthesis,nanocomposite preparation,and characterization

Cellulose nanocrystals (CNCs) are ideal reinforcing agents for polymer nanocomposites because they are lightweight and nano‐sized with a large aspect ratio and high elastic modulus. To overcome the poor compatibility of hydrophilic CNCs in non‐polar composite matrices, we grafted poly(methyl methacrylate) (PMMA) from the surface of CNCs using an aqueous, one‐pot, free radical polymerization method with ceric ammonium nitrate as the initiator. The hybrid nanoparticles were characterized by CP/MAS NMR, X‐ray photoelectron spectroscopy, infrared spectroscopy, contact angle, thermogravimetric analysis, X‐ray diffraction, and atomic force microscopy. Spectroscopy demonstrates that 0.11 g/g (11 wt %) PMMA is grafted from the CNC surface, giving PMMA‐g‐CNCs, which are similar in size and crystallinity to unmodified CNCs but have an onset of thermal degradation 45 °C lower. Nanocomposites were prepared by compounding unmodified CNCs and PMMA‐g‐CNCs (0.0025–0.02 g/g (0.25–2 wt %) loading) with PMMA using melt mixing and wet ball milling. CNCs improved the performance of melt‐mixed nanocomposites at 0.02 g/g (2 wt %) loading compared to the PMMA control, while lower loadings of CNCs and all loadings of PMMA‐g‐CNCs did not. The difference in Young's modulus between unmodified CNC and polymer‐grafted CNC composites was generally insignificant. Overall, ball‐milled composites had inferior mechanical and rheological properties compared to melt‐mixed composites. Scanning electron microscopy showed aggregation in the samples with CNCs, but more pronounced aggregation with PMMA‐g‐CNCs. Despite improving interfacial compatibility between the nanoparticles and the matrix, the effect of PMMA‐g‐CNC aggregation and decreased thermal stability dominated the composite performance.     

Crosslinked carboxylated SBR composites reinforced with chitin nanocrystals

This study aims to develop and characterize the nanocomposites using sulfur cross-linked carboxylated styrene-butadiene rubbers (S-xSBR) as the matrix and chitin nanocrystals (CNCs) as nanofillers. The composites’ morphology and properties were examined by light transmittances, fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD), dynamic mechanical analysis (DMA), thermo gravimetric analyzer (TGA), and tensile properties determination. The addition of CNCs has slight effect on transparency of the composite films. FTIR data confirm the interfacial interactions between CNCs and S-xSBR via hydrogen bonds. CNCs are uniformly dispersed in the matrix from SEM result. The addition of CNCs can significantly improve the tensile strength and modulus both in static and dynamic states. The tensile modulus and tensile strength of S-xSBR/CNCs composites with the 4 wt.% CNCs is 62.5 % and 97.6 % higher than that of pure S-xSBR. The storage modulus, glass transition temperature, and the thermal stability of the composites are higher than those of the neat S-xSBR. The mechanical properties of the composite films are water-responsive, as the swollen samples exhibit obviously decreased strength and modulus. The greatest mechanical contrast is shown in the S-xSBR/CNCs composites with 2 wt.% CNCs loading whose tensile modulus decrease from 60.4 to 6.1 MPa after swelling equilibrium. The significant reinforcement effect of CNCs on S-xSBR is attributed to the unique structure of CNCs and the interfacial interactions in the composite.     

Aramid nanofiber reinforced nitrile rubber assisted by cellulose nanocrystals

A fiber-reinforced rubber composite was prepared by mixing aramid nanofibers (ANF) suspension and nitrile rubber (NBR) latex. The effects of ANF content and corresponding surface modification on the microstructure, vulcanization performance, processing and mechanical properties of composite materials, were systematically investigated. We found that, compared with commonly used short-cut aramid fibers, ANF fillers tend to form a stronger filling network within NBR matrix, resulting in a pronounced Payne effect. By improving the interfacial adhesion via dopamine (DA) coating onto ANF surface, the tensile strength can be further enhanced as expected. Besides, to eliminate the detriment of mechanical performance due to residual sodium polyacrylate in the course of flocculation, cellulose nanocrystal (CNC) was adopted to serve as a thickener during solution mixing. The incorporation of CNC can significantly improve the mechanical properties, which identifies a synergistic reinforcement effect arising from the cooperation of two types of fillers.     

Cellulose acetate ultrafiltration membranes reinforced by cellulose nanocrystals: Preparation and characterization

Cellulose nanocrystals (CNCs) were used as a sustainable additive to improve the hydrophilicity, permeability, antifouling, and mechanical properties of blend membranes. Different CNC loadings (0–1.2 wt %) in cellulose acetate (CA) membranes were studied. The blend membranes were prepared by a phase‐inversion process, and their chemical structure and morphological properties were characterized by attenuated total reflectance–Fourier transform infrared spectroscopy, scanning electron microscopy, porosity, and mean pore size and contact angle measurement. The blend membranes became more porous and more interconnected after the addition of CNCs. The thickness of the top layer decreased and a few large holes in the porous substrate appeared with increasing CNC loading. In comparison with the pure CA membranes, the pure water flux of the blend membranes increased with increasing CNC loading. It reaches a maximum value of 76 L m^?2 h^?1 when the CNC loading was 0.5 wt %. The antifouling properties of the CA membrane were significantly improved after the addition of CNCs, and the flux recovery ratio value increased to 68% with the addition of 0.5 wt % CNCs. In comparison with that of the pure CA membranes, the tensile strength of the composite membranes increased by 47%. This study demonstrated the importance of using sustainable CNCs to achieve great improvements in the physical and chemical performance of CA ultrafiltration membranes and provided an efficient method for preparing high‐performance membranes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43946.     

Surfactant-Assisted Dispersion of MWCNTs in Epoxy Resin Used in CFRP Strengthening Systems

The epoxy resin used as the bonding agent in carbon fiber-reinforced polymer (CFRP) strengthening systems was modified by the infusion of multiwalled carbon nanotubes (MWCNTs). Two types of surfactants, Triton X-100 and C₁₂E₈, were used to disperse the nanotubes in the epoxy resin employing ultrasonic mixing. Dynamic mechanical analysis and tensile tests were conducted to study the effect of the surfactant-assisted dispersion of nanotubes on the thermal and mechanical properties of epoxy composites. The morphology of the epoxy composites was interpreted using scanning electron microscopy (SEM). Moreover, the effect of surfactant treatment on the structure of nanotubes was investigated by Fourier transform infrared (FT-IR). Based on the experimental results, the tensile strength and the storage modulus of the epoxy resin were increased by 32% and 26%, respectively, by the addition of MWCNTs. This was attributed to the homogeneous dispersion of nanotubes in the epoxy resin according to the SEM images. Another reason for the enhancement in the tensile properties was the reinforced nanotube/epoxy interaction as a result of the surfactant anchoring effect which was proved by FT-IR. A moderate improvement in the glass transition temperature (T _g) was recorded for the composite fabricated using Triton X-100, which was due to the restricted molecular motions in the epoxy matrix. To characterize the temperature-dependent tensile behavior of the modified epoxy composites, tensile tests were conducted at elevated temperatures. It was revealed that the MWCNT modification using surfactant substantially improves the tensile performance of the epoxy adhesive at temperatures above the T _g of the neat epoxy.     

Functionalised multi‐walled carbon nanotubes for epoxy nanocomposites with improved performance

BACKGROUND: Carbon nanotubes (CNTs) are fast becoming key components in the production of high‐strength composite materials. Two methods to prepare nanocomposites by covalent bonding between an epoxy matrix and functionalised CNTs that acted as cross‐linkers during polymerisation were investigated. RESULTS: In the standard method, 1 wt% functionalised CNTs was dispersed in epoxy, hardener was added and the composite was cured. In the masterbatch approach, 1 wt% functionalised CNTs was mixed with epoxy in the presence of triethylamine accelerator, then cured. This yielded partially cured epoxy; additional hardener was required to achieve complete curing. Improvements were observed in storage modulus (E′), flexural modulus (E_B), wear resistance and hardness. Thermal stability did not change appreciably for samples prepared by either the standard or masterbatch methods. Variations in the results obtained as a function of preparation method, functionalised CNTs and hardener used are discussed. CONCLUSION: Epoxy nanocomposites having improved mechanical properties were obtained by incorporating functionalised CNTs. Better interaction between the epoxy and CNT was achieved using the masterbatch method; this was attributed to covalent bonding between the CNTs and epoxy. However, optimisation of the CNTs, accelerator and hardener used in composite preparation is required to obtain improved physical properties. Copyright © 2009 Society of Chemical Industry     

Improvement of interfacial adhesion between oxide ceramic nanoparticles and epoxy resin by wet-jet milling

Although ceramic/polymer composites are useful for various applications, such as sensors, electronics, automobiles, and aerospace, the aggregation of nanoparticles can lead to the degradation of the mechanical and functional properties of the composites. To mitigate this, the interfacial adhesion between epoxy resin and the oxide ceramic nanoparticles γ-aluminum oxide (Al₂O₃), silicon dioxide, and magnesium oxide was strengthened by wet-jet milling (WJM) treatment without a chemical modifier. The WJM treatment of the slurry containing nanoparticles and epoxy resin led to the good adsorption of epoxy resin onto the nanoparticle surface, which significantly improved the mechanical properties of the composites. Throughout this process, the amount of epoxy resin adsorbed on the nanoparticle surface and the composite mechanical properties increased with increasing WJM processing pressure, owing to the increased contact between the nanoparticles and epoxy resin droplets and the reduced droplet size. Furthermore, poor solvent was found to be effective for the dispersal of the nanoparticles because the epoxy resin droplets in the slurry were more stable on the nanoparticle surfaces than those in the solvent. When Al₂O₃ nanoparticles were used as a filler, the amount of epoxy resin adsorbed increased from 3.7 to 70.6 mg g⁻¹, and the composite tensile strength increased from 67.1 to 100.3 MPa in poor solvent and under high WJM processing pressure. This optimized WJM treatment will lead to improvements in the mechanical and functional properties of various composite materials.     

Melt processing of polyamide 12 and cellulose nanocrystals nanocomposites

The fabrication of nanocomposites of polyamide 12 (PA12) and cellulose nanocrystals (CNCs) isolated from cotton and tunicates is reported. Through a comparative study that involved solution‐cast (SC) and melt‐processed materials, it was shown that PA12/CNC nanocomposites can be prepared in a process that appears to be readily scalable to an industrial level. The results demonstrate that CNCs isolated from the biomass by phosphoric acid hydrolysis display both a sufficiently high thermal stability to permit melt processing with PA12, and a high compatibility with this polymer to allow the formation of nanocomposites in which the CNCs are well dispersed. Thus, PA12/CNC nanocomposites prepared by melt‐mixing the two components in a co‐rotating roller blade mixer and subsequent compression molding display mechanical properties that are comparable to those of SC reference materials. Young's modulus and maximum stress could be doubled in comparison to the neat PA12 by introduction of 10% (CNCs from tunicates) or 15% w/w (CNCs from cotton) CNCs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42752.     

Novel transparent lignin/epoxy resin nanocomposite film development with enhanced UV resistance performance

The increased importance of nanostructured lignin exhibits a plethora of multifunctional groups, making it a highly adaptable material basis with significant potential for use in the rapidly expanding composite sector. Initially, lignin nanoparticles (NPs) were incorporated into epoxy networks via an in situ polymerization process where isophorone diamine functioned as a hardener to produce lignin/epoxy resin nanocomposites (lignin@TN). These lignin@TN exhibit promising characteristics, enhancing UV-blocking efficiency (35.69% at 404 nm with 3.72 wt% lignin) in transparent nanocomposite films while simultaneously improving mechanical properties compared with neat epoxy resin. Adding lignin nanoparticles to epoxy nanocomposites boost up tensile strength by an impressive 24% compared with the pristine epoxy but it affected the glass transition temperature (T_g). After all, this study has opened new possibilities for utilizing renewable, sustainable, and economic lignin feedstocks as a potential ingredient for reinforcing with thermoset polymers like epoxy.     

Functionally Graded Polyurethane/Cellulose Nanocrystal Composites

The preparation and investigation of functionally graded polymer nanocomposites, which have a concentration gradient of cellulose nanocrystals (CNCs) along one direction, is reported here. As a test bed, a series of nanocomposites consisting of a thermoplastic polyurethane (PU) and 0–15% w/w CNCs is prepared via solvent casting and the mechanical properties of films of these materials are characterized by dynamic mechanical analyses and tensile tests. The formation of graded materials is accomplished by lamination of films with varying CNC content. The processing conditions are optimized to achieve intimate fusion of the individual layers. The elimination of internal interfaces is evidenced by an elongation at break of up to 500%. In order to explore potential applications of graded PU/CNC nanocomposites, structure‐dependent actuation in response to water is demonstrated in a bioinspired architecture. In addition, the damping behavior of cylindrical shaped composites is investigated by way of compression tests. The results show that functionally graded PU/CNC composites show good damping behavior over a much larger range of forces than the neat PU or the homogeneous nanocomposites.     

Crosslinked PVA nanofibers reinforced with cellulose nanocrystals: Water interactions and thermomechanical properties

Acid‐catalyzed vapor phase esterification with maleic anhydride was used to improve the integrity and thermo‐mechanical properties of fiber webs based on poly(vinyl alcohol), PVA. The fibers were produced by electrospinning PVA from aqueous dispersions containing cellulose nanocrystals (CNCs). The effect of esterification and CNC loading on the structure and solvent resistance of the electrospun fibers was investigated. Chemical characterization of the fibers (FTIR, NMR) indicated the formation of ester bonds between hydroxyl groups belonging to neighboring molecules. Thermomechanical properties after chemical modification were analyzed using thermal gravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis. An 80% improvement in the ultimate strength was achieved for CNC‐loaded, crosslinked PVA fiber webs measured at 90% air relative humidity. Besides the ultra‐high surface area, the composite PVA fiber webs were water resistant and presented excellent mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40334.     

Dispersion,Mechanical and Thermal Properties of Epoxy Resin Composites Filled with the Nanometer Carbon Black

The nanometer carbon black (CB) was employed to prepare epoxy resin/carbon black (EP/CB) composites by blending-casting method. The different modified methods of silicone coupling agent were used to improve the dispersion of CB in epoxy resin. The mechanical and thermal properties of EP/CB composites were investigated. Experimental results showed that the mechanical properties increased at first, but decreased with excessive addition of CB. When the mass fraction of CB was 2%, the mechanical properties were maximum. The use of modified CB significantly enhanced the mechanical properties of the composites. For given CB loading, the CB modified by pretreatment method displayed better dispersion in the epoxy resin than that of the direct mixing method. SEM observation revealed that the tensile fracture surface of the composite filled with 2 wt% modified CB held more microcracks than that of 5 wt% modified CB, and the formed microcracks could consume more energy of rupture, finally to have better tensile strength. DSC analysis showed that the glass transition temperature (Tg) of the composites increased with the increasing mass fraction of CB.     

Preparation and mechanical properties of green epoxy nanocomposites with cellulose nanocrystals

Epoxy resin is widely used to make composites, electronic and electric parts, adhesives, and coating materials because it has excellent thermal, electrical, and mechanical properties. Using natural materials in making epoxy composites and nanocomposites would make the final products greener. Therefore, in this study, epoxidized soybean oil (ESO) and cellulose nanocrystals (CNCs) were used to make green epoxy nanocomposites. ESO was prepared by epoxidation of soybean oil with peroxyacetic acid and it was confirmed by Fourier transform infrared spectroscopy. The ESO was mixed with diglycidyl ether of bisphenol A at different weight ratios (10%–50%) and the stoichiometric amount of ethylene diamine was used for curing. CNC content in the nanocomposites was changed from 0.125 to 1 phr. Mechanical properties of the epoxy samples and the nanocomposites were investigated by universal testing machine and izod impact tester. The epoxy sample showed best mechanical properties at ESO 30%. The nanocomposite with CNC 0.25 phr showed best mechanical properties. Fracture surfaces of the epoxy sample and the nanocomposites were investigated by scanning electronic microscope. POLYM. ENG. SCI., 60:439–445, 2020. © 2019 Society of Plastics Engineers