Polyurethanes represent extraordinarily versatile polymeric materials which can be tailored to meet the highly diversified demands of modern technologies such as coatings, adhesives, reaction injection molding, fibers, foams, rubbers, thermoplastic elastomers, and composites. [1±3] It is an important objective in polyurethane development to improve resistance against mechanical deformation without sacrificing high elongation at break. Since the early pioneering advances by Otto Bayer, [4] polyurethane properties are modified either by varying polyurethane microstructure, resulting from step-growth polymerization of isocyanate resins with polyols, or by dispersing inorganic and organic fillers within the polyurethane continuous matrix. A wide variety of fillers, including clay and wollastonites, are being applied in polyurethane formulations to reduce costs and to reinforce the polyurethane matrix. [5] Properties of filled polyurethanes are dependent upon filler shape, average diameter and interfacial coupling. Anisotropic particles with a large length/diameter ratio (aspect ratio), e.g., whiskers and fibers, were found to be especially effective in matrix reinforcement. Frequently, filler addition is accompanied by increased strength and stiffness at the expense of substantially reduced elongation at break.Although nanoscale anisotropic particles are expected to offer attractive potential for polyurethane reinforcement, application of anisotropic nanofillers is restricted by the limited availability of nanowhiskers, poor dispersion, and handling problems associated with potential health hazards resulting from inhalation of such particles. Therefore, several attempts have been made to produce nanowhisker dispersions in polyols which are much easier to handle. For example, N-(4-aminobenzoyl)-caprolactam was polymerized at 200 C in dihydroxy-terminated poly(tetrahydrofuran) to produce dispersions of rigid polybenzamide nanowhisker of approximately 2000 nm length and 200 nm diameter. Interfacial coupling and steric stabilization were achieved by reacting hydroxy end groups of the polyol with N-acyl-lactam end groups located at the polybenzamide whisker surface. Upon curing with diisocyanate, the resulting polyurethane nanocomposites exhibited the unusual combination of increased tensile strength and Young's modulus without sacrificing high elongation at break. [6,7] Particle formation via sol/gel technology leads to dispersions of mainly isotropic nanoparticles in polyurethane. [8] Recently, intercalation of organophilic layered silicates with polymers has been introduced as an attractive route leading to versatile polymer nanocomposites which contain nanoscale layered silicates with high aspect ratio. Layered silicates, e.g., clay, consist of anionically charged layers of aluminum/magnesium or magnesium/lithium silicates where cations such as sodium, potassium, magnesium or calcium are located in the interlayer galleries. They are rendered organophilic when gallery cations are exchanged by quaternary alkyl ammonium...
SUMMARY: Basic correlations between polymer morphology, silicate superstructures, glass temperature, stiffness and toughness of thermoset nanocomposites were investigated as a function of layered silicate type and content. The nanocomposites were based upon hexahydrophthalic anhydride-cured bisphenol A diglycidyl ether and layered silicates such as synthetic fluoromica (Fmica), purified sodium bentonite and synthetic hectorite, all of which were rendered organophilic by means of ion-exchange with various mono-and difunctional alkyl ammonium ions. Enhanced toughness was associated with the formation of dispersed anisotropic laminated nanoparticles consisting of intercalated layered silicates. Nanocomposite superstructures were imaged by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM).
IntroductionEpoxy resins are well known as thermally and environmentally stable materials with applications ranging from adhesives to coatings and matrix resins of composites. Anhydride-cured epoxy resins are key components of electrical insulators. Important objective in epoxy resin chemistry is to improve the toughness/stiffness/strength balance without sacrificing processability, dimensional stability and electrical properties. [1] While microfillers are being used extensively to modify epoxy resin properties, the use of nanofillers is rather limited because of dispersion problems and viscosity build-up relating to strong interparticle interactions of nanofillers. [2] Moreover, nanofillers require special handling due to health hazards relating to inhalation problems. Therefore, it is a very attractive approach to generate nanofillers in-situ. For example, water swellable layered silicates are rendered organophilic via cation exchange of intergallery sodium cations for alkylammonium cations to produce much smaller intercalated and exfoliated nanoparticles. Such in-situ formation of organoclay nanosilicates was reported to afford unusual property combinations such as improved stiffness, strength, thermal stability, flame retardency, and barrier performance. [3][4][5] Although first attempts to exploit organoclay-filled epoxy resins [6] date back to 1965, remarkable progress has been made to tailor epoxy nanocomposite using various organophilic layered silicates. For example, Giannelis and coworkers [7][8][9][10][11][12] applied organoclays modified with bis(hydroxyethyl)methyldodecylammonium chloride in the presence Full Paper: Resin composition, silicate filler modification, and curing agents were varied systematically in order to improve the performance of anhydride-cured epoxy nanocomposites based upon organoclay. The filler component was fluorohectorite which was rendered organophilic by means of cation exchange of intergallery sodium cations for protonated alkylamines with chain lengths variable from butyl (C4), hexyl (C6), octyl (C8), decyl (C10), dodecyl (C12), hexadecyl (C16), octadecyl (C18) to 12-aminododecanoic acid (C12A). The alkyl chain length must exceed six C atoms to afford increased interlayer distances, accompanied by increased stiffness. Compatibilizer addition such as epoxidized and maleinated soy bean oil or dodecenylsuccinate afforded improved tensile strength without sacrificing stiffness and toughness. Morphology development, determined by means of transmission electron microscopy, and fracture behavior were examined. k:/3b2/jobs/eng/280/41_h.3d 26. 7. 2000 Hallal/Witte Young's modulus and tensile strength of nanocomposites containing 10 wt.-% of C12A compatibilized with DDS (wt.-% with respect to total mass of HY 925 and DDS) 42 C. Zilg, R. Thomann, J. Finter, R. Mülhauptof methylnadic anhydride-cured epoxy resin to achieve significant increases of stiffness at low silicate content of 4 wt.-%. However, the performance was found to depend on formulation components and cure conditions. A...
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