Thermal expansion of epoxy-resin/particle composites—a size effect

The thermal expansion of epoxy-resin (Epikote 828)/particle composites has been measured in the range 77 to 450 K. The fillers used were Cu spheres (seven sizes from 5 to 150 μm diameter) and glass ballotini spheres (three sizes from 3.5 to 200 μm diameter). The volume concentrations used were 0.3 and 0.5 for Cu and 0.3 for glass. The experiments show that the addition of filler raises the glass transition temperature T_g, especially for fine particles. Below the normal value of T_g the thermal expansion is independent of particle size while above T_g the expansion is considerably smaller for samples containing the smaller particles. The effect is more pronounced for Cu than for glass filler. In addition a rapid heating rate reduces the expansion for specimens containing smaller particles but it does not effect the expansion for those containing large particles. The results, which are discussed in the light of the work of other authors, suggest that the addition of particles increases T_g by changing the nature of the polymer not only immediately at the particle surface but also for a considerable distance into the polymer itself. This probably occurs because the epoxy bonds strongly to the particles and this inhibits segmental rotations of the polymer even at considerable distances from the particle surface.     

Thermal and mechanical properties of a dendritic hydroxyl‐functional hyperbranched polymer and tetrafunctional epoxy resin blends

Blends of a tetrafunctional epoxy resin, tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM), and a hydroxyl‐functionalized hyperbranched polymer (HBP), aliphatic hyperbranched polyester Boltorn H40, were prepared using 3,3′‐diaminodiphenyl sulfone (DDS) as curing agent. The phase behavior and morphology of the DDS‐cured epoxy/HBP blends with HBP content up to 30 phr were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). The phase behavior and morphology of the DDS‐cured epoxy/HBP blends were observed to be dependent on the blend composition. Blends with HBP content from 10 to 30 phr, show a particulate morphology where discrete HBP‐rich particles are dispersed in the continuous cured epoxy‐rich matrix. The cured blends with 15 and 20 phr exhibit a bimodal particle size distribution whereas the cured blend with 30 phr HBP demonstrates a monomodal particle size distribution. Mechanical measurements show that at a concentration range of 0–30 phr addition, the HBP is able to almost double the fracture toughness of the unmodified TGDDM epoxy resin. FTIR displays the formation of hydrogen bonding between the epoxy network and the HBP modifier. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 417–424, 2010     

Thermal and infrared spectroscopic characterization of historical mortars

The characterization of historical mortars was performed by thermal analysis (TG-DTG), simultaneous infrared spectroscopy (TG-FTIR) and inductively coupled plasma atomic emission spectrometry (ICP-AES). The samples were taken from St. John Church (Tartu, Estonia), built in the 13th–14th centuries. The analyses are important for the restoration of the church.

In reality, mortar is a very difficult system, the lime is accompanied with different hydraulic components. TG-DTG analysis and FTIR methods can be used to identify various components of mortar and to observe the reactions associated with the controlled heating at 25–900°C in dynamic air and nitrogen atmosphere. The elemental composition of the acid-soluble components (ASC) was determined by using the ICP-AES techniques.     

Flame retardancy of fibre-reinforced epoxy resin composites for aerospace applications

The applicability of phosphorus-containing reactive amine, which can be used in epoxy resins both as crosslinking agent and as flame retardant, was compared in an aliphatic and an aromatic epoxy resin system. In order to fulfil the strong requirements on mechanical properties of the aircraft and aerospace applications, where they are mostly supposed to be applied, carbon fibre-reinforced composites were prepared. The flame retardant performance was characterized by relevant tests and mass loss type cone calorimeter. Besides the flame retardancy, the tensile and bending characteristics and interlaminar shear strength were evaluated. The intumescence-hindering effect of the fibre reinforcement was overcome by forming a multilayer composite, consisting of reference composite core and intumescent epoxy resin coating layer, which proved to provide simultaneous amelioration of flame retardancy and mechanical properties of epoxy resins.     

Fractography of a phenol—formaldehyde polymer

Samples of a phenol-formaldehyde polymer with a deliberately introduced flaw were fractured in tension. The appearance of the fracture surface near the flaw suggested the disruption of particles pre-existing in the polymer. At a greater distance from the flaw, a featureless surface was observed which was succeeded by one showing interference colors. At still greater distances, linear features were observed to be lying in the direction of crack propagation which, in most areas, were regular and evenly spaced. There were indications that these features were formed by the curling of a surface film. The above observations are interpreted as providing evidence that tensile fracture is accompanied by plastic defórmation at the fracture surface.     

Thermal analytical investigation of ancient mortars from gothic churches

The use of thermal analysis in studying ancient mortars in English cathedrals is explained. Thermal analysis can be used to investigate both mortar and stone in dated structures. Analysis of ancient mortars show that though recarbonated, they remain soft, yielding to structural deformations. The use of hard (cement mortar) in modern renovation can result in micro-cracking in the stone and subsequent chemical attack from the atmosphere. Contrary to the literature, data developed in the present study suggests that most medieval mortars have reached a near total state of recarbonation.     

New polar silane–PEO polyester complexes as polymer electrolytes

Trifunctional carbosilanes containing hydroxy and cyano groups have been synthesized in good yields and incorporated into polyether-based electrolytes. The new linear and cross-linked modified PEOs have been characterized by DSC and conductivity measurements. The effect of silane content, the length of the PEO block, glycerol concentration, and temperature on glass transition temperature and conductivity of lithium salts complexes of these materials has been evaluated. The new materials showed improved conductivity (∼ 10⁻⁵ S cm⁻¹) at ambient temperatures compared with unmodified polyethers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1093–1106, 1998     

A review on flammability of epoxy polymer,cellulosic and non‐cellulosic fiber reinforced epoxy composites

Natural fiber is well‐known reinforcement filler in polymer‐matrix composites. Composite components like organic polymers and natural fibers are natural fire conductors as the natural fiber consists of cellulose, hemicellulose, and lignin, and hence are as highly flammable as wood. Natural fiber reinforced composite materials are progressively being used in a variety of applications where their fire response is a hazardous consideration, for example, in the automotive (transportation) and building‐construction industries. As a result, an awareness of their performance or response during a fire and the use of conventional fire retardants are of great importance, as they are subject to thermal decomposition when exposed to intensive high heat or fire sources. In this review paper, fire flammability is the main concern for cellulosic and non‐cellulosic fiber‐reinforced polymer composites, especially epoxy composites. This paper reviews the literature on the recent developments in flammability studies concerning polymers, epoxy polymers, cellulosic‐fibers, and non‐cellulosic fiber‐reinforced epoxy bio‐composites. The prime objective of this review is to expand the reach of “fire retardants for polymer materials and composites” to the science community, including physicists, chemists, and engineers in order to broaden the range of their applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermal stability of epoxy adhesives

A study was made of the thermal stability of epoxy compounds which were unfilled or contained metallic fillers such as aluminium dust, aluminium flakes, powdered bronze, powdered brass and silver flakes. The properties of the compounds were modified by the use of various hardeners.

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Zusammenfassung Es wurde die thermische StabilitÄt ungefüllter und gefüllter Epoxidkompositionen untersucht. Als metallische Füllstoffe wurden Aluminiumpulver und -flocken, Messing- und Bronzpulver sowie Silberflocken eingesetzt. Die Eigenschaften dieser Kompositionen wurden mit verschiedenen HÄrtern modifiziert.

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Dielectric relaxation processes in epoxy resin—ZnO composites

Polymer matrix‐ZnO microcomposites were prepared in different filler concentrations. The electrical relaxation dynamics of all samples was examined by means of broadband dielectric spectroscopy (BDS) over a wide temperature range. Two relaxation modes (namely β and γ), observed in the low temperature region, are attributed to the reorientation of small polar groups of the polymer matrix. Glass‐rubber transition (α‐mode) of the polymeric matrix and interfacial polarization phenomena are considered as responsible for the recorded relaxation processes in the high temperature region. An additional relaxation mode, named intermediate dipolar effect (IDE), is recorded at temperatures higher than ?30 °C in all composites. Its occurrence and dynamics are related to the presence and concentration of the filler. IDE and α‐relaxation are observed in the same frequency and temperature range, leading to a mutual superposition. The two processes were distinguished following a simulation procedure employing the simultaneous fitting of two Havrilliak‐Negami terms and a third term describing the contribution of DC conductivity to dielectric losses. The temperature dependence of relaxation times for α‐mode follows the Vogel‐Tamann‐Fulcher equation, whereas IDE relaxation times follow unusual temperature dependence. The latter is discussed under the assumption of intrinsic interfacial polarization phenomena within ZnO crystal domains. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 445–454, 2009     

Positron annihilation in amine-cured epoxy polymers—pressure dependence

Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ₃ and formation probability I₃ decreased with increasing pressure. The decrease in τ₃ is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm³. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.     

Mode-I interlaminar fracture behaviour of nanoparticle modified epoxy/basalt fibre-reinforced laminates

The effect of nano-reinforcements on fracture behaviour of bulk epoxy nanocomposites and mode-I interlaminar fracture toughness of filament-wound basalt fibre-reinforced laminates was studied. Fracture energy of the bulk epoxy nanocomposites significantly increased with acrylic tri-block-copolymer addition but remained unchanged with incorporation of nanoclay. Delamination fracture toughness was not influenced by the presence of nanoparticles in the matrix. Decreasing fibre volume fraction, on the other hand, significantly improved interlaminar fracture energy. Rigid fibres in these composites constrict the stress field ahead of the crack-tip. Hence, increasing resin content enhanced composite delamination energy by increasing the capacity for matrix deformation. Interlaminar crack propagation through the composite was observed to occur mainly by interfacial failure and matrix cracking.     

Effect of new hyperbranched polyester of varying generations on toughening of epoxy resin through interpenetrating polymer networks using urethane linkages

In this study, a previously unreported methodology is attempted to improve the inherent brittleness in diglycidyl ether of bisphenol-A based epoxy resin using hyperbranched polymers as toughening agents. Four different hyperbranched polyesters (HBPs) with increasing generations (1–4, denoted as HBP-G1 to HBP-G4) were synthesized by reacting calculated amount of dipentaerythritol (used as a core) and dimethylol propionic acid (AB₂ type monomer) through pseudo one-step melt polycondensation method. The newly synthesized HBPs were characterized using spectral, thermal and physical measurements, which confirmed the formation of highly branched structure and decreasing thermal stability with increasing HBP generations. Further, toughening of the epoxy resin is carried out by reacting each generation of the HBP with epoxy using hexamethylene diisocyanate as an intermediate linkage resulting in the formation of HBP-Polyurethane/Epoxy-g-Interpenetrating Polymer Networks (HBP-PU/EP-g-IPNs). A linear polyol-PU/EP-g-IPN is also synthesized for the purpose of comparison. It is found that the HBP modified epoxy samples exhibited higher toughness in comparison to that of neat epoxy and linear polyol based epoxy samples. On the other hand, flexural properties, thermal stability and glass transition temperature of the modified samples is lower than neat epoxy sample due to the existence of flexible urethane linkages and decrease in the cross-linking density of epoxy matrix. The toughening characteristics exhibited by the HBPs are corroborated from the existence of heterogeneous morphology using SEM data.     

Elastification of heat-resistant epoxy polymer matrices

The behavior of fracture toughness parameters in comparison with other physicomechanical parameters of failure was examined with the aim to evaluate the efficiency of modification (elastification) of heat-resistant binders, with four epoxy-amine compounds taken as examples.     

Thermal transitions and polymer/polymer miscibility

Poly(vinyl chloride) was blended with ethylene-vinyl acetate copolymer containing 70 wt% of vinyl acetate. The system shows a single glass transition temperature for all compositions, indicating their miscibility. TheT _g vs. composition curves display an inflection, which changes with the chemical environment of the initial solution. The best fit to the shape of the curve was well reproduced by the Kovacs-Braun equation. The T _g values reveal local heterogeneity, which means no total miscibility at a molecular level. Negative values of the Flory-Huggins interaction parameter were obtained from the calorimetric data.Support for this research by CNP_q, FINEP and CAPES is appreciated.     

Role of polyester resin on the epoxy matrix system

Interpenetrating polymer network (IPN) as a polymer blend matrix was prepared using different ratios of polyester (PE) to epoxy (E) resins. It was observed that 11.33 wt % PE in the blend provides maximum mechanical properties, particularly the modulus for the casting, and such a blend is referred to as “optimum casting” (OC). The density of the blends containing up to 11.33% PE, especially of OC, was found to have increased by the presence of PE in the E network significantly more than would have been estimated from the calculation based on the rule of additivity. The thermal expansion coefficient parameter (Δα_i) of the OC was found to be 2.68 × 10^?4K^?1. The scanning electron micrograph (SEM) of the fracture surface of OC only showed a glass-like smooth surface. The change in the mechanical properties due to the varying proportions of PE in the blend was studied critically on the basis of failure mechanisms and morphological features. The differential thermogravimetric analysis (DTGA) of OC indicated only one peak in spite of the presence of two resin constituents, whereas castings with an excess of PE in the blend showed multiple peaks. Interestingly, as far as the DTGA is concerned, a domain of seemingly OC origin was found to remain, even in the blends containing PE resin above its optimum level (> 11.33 wt %). ©1995 John Wiley & Sons, Inc.     

Thermal expansion of polyoxymethylene

The thermal expensivities of polyoxymethylene crystals in the direction parallel (α_|^c) and perpendicular (α_‖^c) to the chain axis have been measured from 160 to 400 K using wide-angle x-ray diffraction. Although polyoxymethylene has a helical chain structure, it exhibits a thermal expansion behavior similar to that of polymer crystals with planar zigzag chains, namely that α_‖^c is negative while α_|^c is positive and larger by an order of magnitude. The negative α_‖^c arises from the shortening along the chain axis caused by the torsional and bending motions of the chain, whereas the large and positive α_|^c reflects the weak interaction across the chains. Combining the crystal data with dilatometric measurements on semicrystalline samples, the thermal expansivity is found to vary linearly with crystallinity, thus allowing the expansivity of the amorphous phase to be derived by extrapolation. With the thermal expansivities of the crystalline and smorphous phases known, the draw ratio dependence can be calculated in terms of existing models and is found to agree reasonably with experimental data.     

Thermal and X‐ray powder diffraction studies of aliphatic polyester dendrimers

The syntheses and thermal and X‐ray powder diffraction analyses of three sets of aliphatic polyester dendrimers based on 2,2‐bis(hydroxymethyl)propionic acid as a repeating unit and 2,2‐dimethyl‐1,3‐propanediol, 1,5‐pentanediol, and 1,1,1‐tris(hydroxymethyl)ethane as core molecules are reported. These dendritic polyesters were prepared in high yields with the divergent method. The thermal properties of these biodendrimers were evaluated with thermogravimetric analysis and differential scanning calorimetry. The thermal decomposition of the compounds occurred around 250 °C for the hydroxyl‐ended dendrimers and around 150 °C for the acetonide‐protected dendrimers. In addition, the crystallinity of the lower generation dendrimers was evaluated with X‐ray powder diffraction. The highest crystallinity and the highest melting points were observed for the first‐generation dendritic compounds. The higher generation dendrimers showed weaker melting transitions during the first heating scan. Only the glass‐transition temperatures were observed in subsequent heating scans. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5574–5586, 2004     

Curing behavior and toughening performance of epoxy resins containing hyperbranched polyester

The effects of the hyperbranched polyester with hydroxyl end groups (HBPE‐OH) on the curing behavior and toughening performance of a commercial epoxy resin (diglycidyl ether of bisphenol A, DGEBA) were presented. The addition of HBPE‐OH into DGEBA strongly increased its curing rate and conversion of epoxide group due to the catalytic effect of hydroxyl groups in HBPE‐OH and the low viscosity of the blend at curing temperature. The improvements on impact strength and critical stress intensity factor (or fracture toughness, K_1c) were observed with adding HBPE‐OH. The impact strength was 8.04 kJ m^?1 when HBPE‐OH reached 15 wt% and the K_1c value was approximately two times the value of pure epoxy resin when HBPE‐OH content was 20 wt%. The morphology of the blends was also investigated, which indicated that HBPE‐OH particles, as a second phase in the epoxy matrix, combined with each other as the concentration of HBPE‐OH increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Cure kinetics of epoxy resin and thermoplastic polymer

The cure kinetics and morphology of diglycidyl ether of bisphenol A (DGEBA) modified with polyvinyl acetate (PVAc) using diaminodiphenylmethane (DDM) as hardener were investigated through differential scanning calorimetry (DSC) and environmental scanning electron microscopy (ESEM). Isothermal curing measurements were carried out at 150, 120 and 80°C. The kinetic parameters were obtained using the general autocatalytic chemically controlled model. The comparison of the kinetic data indicates that the presence of PVAc does not change the autocatalytic nature of the cure reaction. Two T _g’s were observed in the fully cured samples of the modified systems. ESEM micrographies confirm the biphasic morphology.