Morphology and rheology relationships of epoxy/core‐shell particle blends

The morphological and rheological behaviors of toughened epoxy resins modified with core‐shell rubber particles (CSR) were studied. These rubber particles were based on a poly (butadiene‐co‐styrene) core and a crosslinked poly (methyl methacrylate) shell. The effect of functionalized groups was performed on two types of CSR particles: first, those containing carboxyl‐functionalized groups (CSf), and second, particles containing no carboxyl‐functionalized groups (CSnf) in the PMMA‐shell. For these blends, the correlations between the morphology, particle dispersion state and their rheological behaviors before curing were investigated. Preliminary work using TEM micrographs indicated that the blends modified with CSf and CSnf exhibited the same particle size but differed with respect to the dispersion state. Rheological behavior of these blends was assessed in steady shear flow and dynamic viscoelastic experiments. Yield viscosity near‐zero shear rate occurred in the DGEBA/CSf blend presenting non‐Newtonian behavior at the particle volume fraction of 20% vol. The rheological behavior was clearly related to the state of particle dispersion and analyzed taking into account interactions between the particles‐particles and the particles‐matrix. The Williams‐Landel‐Ferry (WLF) shift procedure was used to construct modulus master curves G′ and G″ from the elastic solid state to molten polymers. A secondary plateau existed at low frequencies and was related to the presence of interactions leading to a physical network‐type structure. The deviation between theoretical G′ (Paleirne's model) and experimental G′ values was evaluated and exhibited high elasticity at the terminal zone, which correlated well with available literature.     

Simultaneously improved toughness and dielectric properties of epoxy/core‐shell particle blends

Epoxy/core‐shell particle blends were prepared using a diglycidylether of bisphenol A epoxy and acrylics‐type core‐shell particles. The impact strength of the blends was tested, and the result showed that the epoxy was greatly toughened with optimum core‐shell particle content. Meanwhile, the dielectric properties of both epoxy and its blends were investigated using a broadband dielectric analyzer. It was found that the dielectric constant of the epoxy blends with lower core‐shell particle content were less than that of the epoxy in the investigated frequency range, while the dielectric loss was less than that of the neat epoxy over a low frequency range, even for the epoxy blends with the optimum core‐shell particle content. The dielectric breakdown strength of the epoxy blends at room and cryogenic temperature were also investigated. To identify the primary relationship of the above properties and structure of the epoxy blends, the microstructure of the core‐shell particle and the morphology of the samples were observed by transmission electron microscopy and scanning electron microscopy. It was considered that these epoxy/core‐shell particle blends with improved toughness and desirable dielectric properties could have a potential application in the insulation of electronic packaging system. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Scratch behavior of epoxy nanocomposites containing α‐zirconium phosphate and core‐shell rubber particles

The effects of nanoscale core‐shell rubber (CSR) particles and α‐zirconium phosphate (ZrP) nanoplatelet fillers on the scratch behavior of epoxy have been examined using a newly established ASTM scratch testing method. The critical load for onset of microcrack formation is utilized to determine scratch resistance of the epoxy nanocomposites. Optical microscopy and scanning electron microscopy were performed to determine failure and fracture patterns caused by the scratch. The findings of this study suggest that the introduction of nanoparticles or nanoplatelets does not necessarily enhance the scratch resistance of epoxy. This implies that increases in ductility and fracture toughness alone, i.e., the epoxy/CSR case, and enhancements in modulus and tensile strength alone, i.e., the epoxy/ZrP case, will not necessarily improve scratch resistance of epoxy matrix. A combination of material property attributes is needed to prepare scratch resistant polymers. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Core–shell particles to toughen epoxy resins. I. Preparation and characterization of core–shell particles

A two-stage, multistep soapless emulsion polymerization was employed to prepare various sizes of reactive core–shell particles (CSPs) with butyl acrylate (BA) as the core and methyl methacrylate (MMA) copolymerizing with various concentrations of glycidyl methacrylate (GMA) as the shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either the core or shell. The number of epoxy groups in a particle of the prepared CSP measured by chemical titration was close to the calculated value based on the assumption that the added GMA participated in the entire polymerization unless it was higher than 29 mol %. Similar results were also found for their solid-state ^13C-NMR spectroscopy. The MMA/GMA copolymerized and EGDMA-crosslinked shell of the CSP had a maximum glass transition temperature (T_g) of 140°C, which was decreased with the content of GMA at a rate of −1°C/mol %. However, the shell without crosslinking had a maximum T_g of 127°C, which decreased at a rate of −0.83°C/mol %. The T_g of the interphasial region between the core and shell was 65°C, which was invariant with the design variables. The T_g of the BA core was −43°C, but it could be increased to −35°C by crosslinking with EGDMA. The T_g values of the core and shell were also invariant with the size of the CSP. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2069–2078, 1998     

Fracture behavior of core‐shell rubber–modified clay‐epoxy nanocomposites

Morphology and fracture mechanisms in two nanoclay‐filled epoxy systems were investigated using both microscopy and spectroscopy tools. Clay exfoliation was achieved using a series of sample preparation steps, and confirmed using wide angle X‐ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Significant improvement in modulus was obtained when clay exfoliation was achieved. Incorporation of core‐shell rubber (CSR) in both caly‐filled epoxy systems leads to greatly enhanced fracture toughness. Optical microscopy and TEM observations of the CSR‐modified nanocomposites suggest that CSR cavitation. shear yielding of the matrix, clay layer delamination. CSR bridging, crack bifurcation. and crack deflection are among the operative toughening mechanisms observed, depending on the nature of the epoxy matrix utilized.     

Toughening of polycarbonate: Effect of particle size and rubber phase contents of the core‐shell impact modifier

The toughening behavior of polycarbonate modified with core‐shell type particles was investigated. The alloys were found to exhibit maximum impact strength upon addition of a modifier with a poly(butyl acrylate) rubbery core of 0.25 μm diameter. The incorporation of particles with diameter greater than 0.25 μm resulted in decreased impact strength. The influence of rubber phase contents on toughness was also studied. It was observed that the alloys exhibited maximum impact strength upon addition of 4 wt % rubber phase. Further increase in the rubber phase content resulted in reduced impact strength. Fractography of the samples showed that, below 4 wt % rubber phase content, the fracture occurs mainly by internal crazing and, from 4 wt % onward, only by shear deformation. When the effect of dual particle size distribution was analyzed, it was found that there was only a moderate increase in toughness compared with alloys containing monosized particles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 748–755, 2005     

Toughening of unsaturated polyester resins with core–shell rubbers

The effects of core–shell rubbers (CSRs) as tougheners on the fracture properties of unsaturated polyester (UP) resins during curing at 110°C are investigated. CSRs were synthesized by two‐stage soapless emulsion polymerizations; the soft core was made from rubbery poly(n‐butyl acrylate), whereas the hard shell was made from methyl methacrylate, ethylene glycol dimethacrylate, and various concentrations of glycidyl methacrylate. Depending on the content of glycidyl methacrylate in the CSR shell and the amount of CSR added to the UP, the fracture properties of the CSR‐toughened UP resins varied. The experimental results are explained by an integrated approach of measurements of the static phase characteristics of a styrene/UP/CSR system, the reaction kinetics, the cured sample morphology, the glass‐transition temperatures, and the fracture toughness with differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, and dynamic mechanical analysis. Finally, the toughening mechanism for the CSR‐toughened UP resins is also explored. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Toughening of polyvinylchloride by methyl methacrylate–butadiene–styrene core–shell rubber particles: Influence of rubber particle size

An analysis was made on the effects of rubber particle size on the mechanical properties and deformation mechanisms of transparent polyvinyl chloride (PVC) blends containing core–shell methyl methacrylate–butadiene–styrene (MBS) impact modifiers. The critical interparticle distance was found not to be the criterion for the brittle‐ductile transition in the blends. In tensile tests, the blends with larger (100–280 nm) rubber particles exhibited intense stress‐whitening, while one blend with small (83 nm) rubber particles showed only slight stress‐whitening. These differences were due to an increase in resistance to cavitation with decreasing rubber particle size. Transmission electron microscopy studies on blends with a bimodal distribution of particle sizes showed that in the whitened zone of Izod specimens the larger rubber particles cavitated and expanded on yielding, while the smaller particles remained intact. However, Izod test results showed that small MBS rubber particles can toughen the PVC matrix very effectively, especially at low temperatures and at low rubber concentrations. The deformation mechanisms responsible for these effects were discussed. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Self‐healing and interfacially toughened carbon fibre‐epoxy composites based on electrospun core–shell nanofibres

Dual components of a self‐healing epoxy system comprising a low viscosity epoxy resin, along with its amine based curing agent, were separately encapsulated in a polyacrylonitrile shell via coaxial electrospinning. These nanofiber layers were then incorporated between sheets of carbon fiber fabric during the wet layup process followed by vacuum‐assisted resin transfer molding to fabricate self‐healing carbon fiber composites. Mechanical analysis of the nanofiber toughened composites demonstrated an 11% improvement in tensile strength, 19% increase in short beam shear strength, 14% greater flexural strength, and a 4% gain in impact energy absorption compared to the control composite without nanofibers. Three point bending tests affirmed the spontaneous, room temperature healing characteristics of the nanofiber containing composites, with a 96% recovery in flexural strength observed 24 h after the initial bending fracture, and a 102% recovery recorded 24 h after the successive bending fracture. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44956.     

Polylactide,nanoclay, and core–shell rubber composites

Three types of composites, namely, polylactide (PLA)/nanoclay, PLA/core–shell rubber, and PLA/nanoclay/core–shell rubber, were melt compounded via a corotating twin‐screw extruder. The effects of two types of organically modified montmorillonite nanoclays (i.e., Cloisite®30B and 20A), two types of core (polybutylacrylate)–shell (polymethylmethacrylate) rubbers (i.e., Paraloid EXL2330 and EXL2314), and the combination of nanoclay and rubber on the mechanical and thermal properties of the composites were investigated. According to X‐ray diffraction and transmission electron microscopy analyses, both types of PLA/5 wt% nanoclay composites had an intercalated morphology. In comparison with pure PLA, both types of PLA/5 wt% nanoclay composites had an increased modulus, similar impact strength, slightly reduced tensile strength, and significantly reduced strain at break. On the other hand, PLA/EXL2330 composites with a rubber loading level of 10 wt% or higher had a much higher impact strength and strain at break, but a lower modulus and strength when compared with pure PLA. The simultaneous addition of 5 wt% nanoclay (Cloisite®30B) and 20 wt% EXL2330 resulted in a PLA composite with a 134% increase in impact strength, a 6% increase in strain at break, a similar modulus, and a 28% reduction in tensile strength in comparison with pure PLA. POLYM. ENG. SCI. 46:1419–1427, 2006. © 2006 Society of Plastics Engineers     

Electrospinning core‐shell nanofibers for interfacial toughening and self‐healing of carbon‐fiber/epoxy composites

This article reports a novel hybrid multiscale carbon‐fiber/epoxy composite reinforced with self‐healing core‐shell nanofibers at interfaces. The ultrathin self‐healing fibers were fabricated by means of coelectrospinning, in which liquid dicyclopentadiene (DCPD) as the healing agent was enwrapped into polyacrylonitrile (PAN) to form core‐shell DCPD/PAN nanofibers. These core‐shell nanofibers were incorporated at interfaces of neighboring carbon‐fiber fabrics prior to resin infusion and formed into ultrathin self‐healing interlayers after resin infusion and curing. The core‐shell DCPD/PAN fibers are expected to function to self‐repair the interfacial damages in composite laminates, e.g., delamination. Wet layup, followed by vacuum‐assisted resin transfer molding (VARTM) technique, was used to process the proof‐of‐concept hybrid multiscale self‐healing composite. Three‐point bending test was utilized to evaluate the self‐healing effect of the core‐shell nanofibers on the flexural stiffness of the composite laminate after predamage failure. Experimental results indicate that the flexural stiffness of such novel self‐healing composite after predamage failure can be completely recovered by the self‐healing nanofiber interlayers. Scanning electron microscope (SEM) was utilized for fractographical analysis of the failed samples. SEM micrographs clearly evidenced the release of healing agent at laminate interfaces and the toughening and self‐healing mechanisms of the core‐shell nanofibers. This study expects a family of novel high‐strength, lightweight structural polymer composites with self‐healing function for potential use in aerospace and aeronautical structures, sports utilities, etc. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Rheological behavior and microstructure of aqueous suspension of carboxylated core‐shell structured latex particle

Core‐shell type carboxylated particles form a flocculated structure in aqueous suspension with neutralization of carboxyl groups. Rheological behaviors of the suspension have been studied at various temperatures, and microstructures of the suspension have been discussed from the rheological behaviors and SAXS measurements. At 25°C, G′ is larger than G″ in all ω regions, and G′ is almost independent of ω, and the diffraction peak is detected by SAXS. These results mean that a three‐dimensional network of interconnected lattice‐like flocculated structure is formed. With increasing temperature, ω dependency of G′ becomes stronger and distance of the particles of the structure does not changed. These mean the network linkage is disrupted partially by thermal motion, and the interconnected lattice‐like flocculated structure changes to an isolated lattice‐like one with increasing temperature. With increasing the degree of neutralization, an isolated structure changes to an interconnected three‐dimensional structure decreasing the thermal motion just like decreasing temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1627–1633, 2001     

Toughening epoxy resins with solid acrylonitrile–butadiene rubber

The feasibility of using solid acrylonitrile–butadiene rubbers (NBR) with 19 and 33% w/w acrylonitrile to toughen diglycidyl ether of bisphenol A (DGEBA) epoxy resins has been investigated. Thermal analysis experiments revealed a two‐phase morphology of these rubber‐modified epoxies. However, the higher content of acrylonitrile in the rubber caused better compatibility between NBR and the epoxy resin. The rubber with 33% acrylonitrile was found to be an effective toughening agent for DGEBA epoxy resins. Fracture surface studies and also the high tensile strength of crosslinked high molecular weight NBR suggest that the toughening effect should arise from rubber bridging and tearing mechanisms. © 2000 Society of Chemical Industry     

Effect of the core‐shell impact modifier shell thickness on toughening PVC

The shell thickness of a core/shell impact modifier is found to be the single most important factor in the toughening of rigid polyvinyl chloride (PVC). When the shell thickness is greater than a critical value of 15.8 nm, these core‐shell elastomeric particles are able to remain structurally intact and well dispersed within the PVC matrix after melt blending. However, too thick a shell thickness results in a hard core (high modulus) of these core/shell particles and loss of the rubbery nature required of an efficient impact modifier. Therefore, these over‐thick particles can act only as rigid fillers, not as efficient rubbery modifiers. On the other hand, when the shell thickness is less than the critical value of 4.9 nm, too thin a shell layer is simply unable to fully protect and cover the inner rubbery core during vigorous processing conditions, and these core‐shell particles tend to connect with one another through the partially exposed core to form a cellular‐like structure, thus resulting in poor toughening efficiency. Regardless of the particle size, as long as the shell thickness of these core/shell elastomers is between these two critical values (15.8 nm and 4.9 nm), they all display high efficiency in toughening rigid PVC. Polym. Eng. Sci. 44:1885–1889, 2004. © 2004 Society of Plastics Engineers.     

Heat resistance of epoxy resins

A torsion pendulum has been found to be a sensitive tool to detect the first steps of pyrolysis. It has been shown that some primary bonds break at elevated temperatures, but in the presence of oxygen, new crosslinks can form simultaneously. In resin heated in air, embrittlement due to these crosslinks starts on the surface of the resin and progresses inwards with time.     

Influence of impact load‐temperature conformity on toughening of poly(vinyl chloride) by core‐shell rubber particles

The work focused on the elucidation of several key parameters in toughening poly(vinyl chloride) (PVC) by the methyl methacrylate–butadiene–styrene (MBS) core‐shell particles. Accordingly, blends containing various weight percent of the MBS particles were prepared and characterized by dilute solution viscometry, dynamic light scattering, dynamic mechanical thermal analysis, transmission electron microscopy, and temperature variable impact test. The results showed PVC/MBS solution miscibility in almost all compositions with their maximum thermodynamic affinities at about 17 and 67 wt % of MBS in tetrahydrofurane (THF). In addition, MBS weight percent increase in its blend with the PVC above 10 led to severe impact energy raise with eventual leveling at about 17 wt %. Furthermore, blend toughness and its components miscibility in solution increased in parallel up to 20 wt % of MBS particles. On the other hand, blend toughness declined with test temperature decrease toward impact modifier core T_g at about ?30°C even for the sample with 20 wt % of the MBS particles. Finally, the brittle‐ductile transition of the blend containing 20 wt % of the MBS particles comparison with its matrix tan δ‐temperature correlation implied 2500 J/m impact energy equivalence with 90°C sample temperature rise in secondary relaxation activation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation of core‐shell latexes for paper coatings

Novel core‐shell latices with a partially crosslinked hydrophilic polymer core and a hard hydrophobic shell of polystyrene were prepared to improve optical properties of coated paper such as gloss and brightness. These core‐shell latices were prepared by sequential addition of a monomer mixture of styrene, n‐butylacrylate and methacrylic acid. Different crosslinkers were used to form the polymer core and in the second stage styrene to form the hard shell component. In addition, attempts were made to further improve optical properties by introducing a new polymerizable optical brightener, i.e., 1‐[(4‐vinylphenoxy)methyl]‐4‐(2‐phenylethylenyl)benzene during polymerization either into the core or into the shell. The prepared core‐shell latex particles were used as specialty plastic pigments for paper coating together with kaolin as the primary pigment. The runability of paper coating formulation by either using a laboratory scale Helicoater or pilot scale JET‐coating machine was very good. The produced coated papers were printed on both sides employing a heat set web offset (HSWO) printer to study the quality of image reproduction in terms of print gloss, print mottle, print through, etc. The core‐shell latices improved the overall print quality. Furthermore, the results demonstrated that by optimizing polymer composition one can significantly enhance the optical properties and surface smoothness of coated paper. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Rubber modification of unsaturated polyester resin with core–shell rubber particles: Effect of shell composition

Poly(butyl acrylate)/poly(vinyl acetate‐co‐methyl methacrylate) PBA/P(VAc‐co‐MMA) core–shell rubber particles with various shell compositions, i.e., VAc/MMA weight ratios, were used to toughen unsaturated polyester. The morphology and surface‐free energy of the rubber particles were determined by transmission electron microscopy (TEM) and contact angle measurements, respectively. The effect of shell structure on the dispersion state of rubber particles inside the unsaturated polyester resin was studied by scanning electron microscopy and TEM. Increasing MMA units in the shell changed the particle dispersion state from small agglomerates or globally well‐dispersed particles to large aggregates in the cured‐resin matrix. For the blends that contain 5 wt% rubber, the highest un‐notched impact toughness, stress‐intensity factor (K_IC), and fracture energy (G_IC) were observed for the blend containing PVAc shell particles. The results showed that by increasing the particle level from 5 to 10 wt%, the highest K_IC and G_IC values were obtained for the blend containing rubber particles with VAc/MMA (80/20 wt/wt) copolymer shell. The crack‐tip damage zone in the neat and rubber‐modified unsaturated polyester resins was observed by means of transmission optical microscopy. In addition, using PVAc shell particles exhibited a minimum reduction in the volume shrinkage and tensile properties of the rubber‐modified resin. POLYM. ENG. SCI., 52:1928–1937, 2012. © 2012 Society of Plastics Engineers     

Dynamic mechanical analysis of rubber toughened epoxy resins

Dynamic mechanical analysis (DMA) was used to characterize cured epoxy resin formulations from ?150°C to temperatures above their α transitions. The resins were aromatic amine and aliphatic amine cured and were modified with carboxylterminated acrylonitrile-butadiene (CTBN) rubbers to improve their toughness, A DuPont 981 dynamic mechanical analyzer was used to measure the modulus and mechanical loss factor (tan δ) of the samples. Changes in the α and β transitions in the scan of tan δ as a function of temperature were related to changes in the formulation. Relations were also sought between changes in the DMA data and fracture and impact toughness of the cured formulations obtained using an instrumented impact test. Impact tests were performed at ?196°C and at room temperature. Results indicate that fracture toughness and the dynamic mechanical properties are affected by the amount of rubber, the compatibility of the rubber and epoxy, and changes in the curing agent stoichiometry.     

Synthesis and self‐healing behavior of thermoreversible epoxy resins modified with nitrile butadiene rubber

Cracks may generate in epoxy resins, which can affect the comprehensive property and shorten service life. The problem is expected to be resolved by endowing epoxy resin with self‐healing performance. Herein, a new kind of self‐healing epoxy resin containing both Diels–Alder (DA) bonds and nitrile butadiene rubber (NBR) has been developed. The self‐healing performance and mechanical properties of as‐prepared epoxy resins are investigated by qualitative observation and quantitative measuring. Results reveal that the as‐prepared epoxy resins exhibit excellent self‐healing performance and multiple repair ability, and the self‐healing behavior is based on dual actions of thermal reversibility of DA reaction and thermal movement of molecular chains. Furthermore, the thermoreversible DA bonds contribute much to the recovery of mechanical property, while the incorporated thermoplastic NBR accelerates the whole healing process. The self‐healing efficiency of epoxy resins can be enhanced markedly by introducing thermoplastic NBR. In addition, the self‐healing epoxy resins also exhibit outstanding reprocessing performance, which makes it possible of recycling waste epoxy resin. POLYM. ENG. SCI., 59:1603–1610 2019. © 2019 Society of Plastics Engineers