Epoxies toughened with triblock copolymers

Three different commercial triblock copolymers from Arkema were evaluated as potential toughening agents for two different lightly crosslinked epoxies. It was found that the plane strain fracture toughness, K_IC, was on the order of 3.0 MPa√m for 10 parts per hundred resin (phr) of NanoStrength™ E20 resin (a styrene–butadiene–methylacrylate, SBM, type triblock copolymer) in epoxies cured with either aminoethylpiperazine or piperidine. In contrast, 10 phr NanoStrength E40 resin (also an SBM type triblock copolymer) was ineffective in toughening such epoxies. The difference in toughening effectiveness was attributed to the smaller amount of polybutadiene present in the E40 resin. The third toughening agent from Arkema was NanoStrength M22 resin, which is a symmetric triblock copolymer consisting of side blocks of PMMA surrounding a center block of poly(butyl acrylate) and is designated as MAM. At 10 phr MAM both lightly crosslinked epoxies exhibited improvements in toughness. Morphologies were characterized using TEM and toughening mechanisms revealed using SEM and TOM. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1470–1481, 2007     

Liquid crystalline epoxides with long lateral substituents: Mechanical and thermal properties

In this work, mechanical and thermal properties of liquid crystalline epoxides (LCEs) with long lateral substituents from 4 to 12 carbon atoms cured with diaminodiphenylmethane were evaluated and analyzed by dynamic mechanical analysis, tensile tests, scanning electron micrographs (SEM), and thermo‐gravimetrical analysis. The experimental results indicated that the Young's modulus and α, β transitions in crosslinked networks are associated with the length of lateral substituents. The plastic deformation in fracture surfaces was observed by SEM. Thermal stability, water and solvent absorption of cured networks was dependent on the length of lateral substituents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2835–2841, 2007     

Use of phase diagrams in the analysis of polymer network formation

A conversion - temperature - transformation (CTT) diagram was used to analyze several non-isothermal processes of polymer network formation: a) range of heating rates where reliable kinetic information can be obtained; b) a novel process involving part of the cure in the glassy state; c) phase separation in rubber-modified epoxies cured in heated molds.     

Oscillatory shear experiments as criteria for potential vitreous substitutes

Polymers of N-vinyl-2-pyrrolidinone were synthesized as potential substitutes for the vitreous body of the eye. Certain polymers displayed typical gel behaviour and they were further characterized by dynamic shear analysis. The mechanical spectra of the fully hydrated samples indicated a covalently crosslinked network system i.e. G′ > G″ and slopes approaching zero. The gels were also subjected to preshearing by injection through a small needle, a procedure that is essential in the use of vitreous substitutes. The polymers crosslinked with ethylene dimethacrylate dramatically changed their characteristics following the injection process, as the values of shear moduli dropped substantially, and the gels behaved like free flowing fluids at higher frequencies (G″ > G′). The gels crosslinked with diallyl ether or divinyl glycol displayed only minor changes in their viscoelastic behaviour, but their moduli were very low before and after injection. The gels that were produced without crosslinking agents maintained their properties as virtually unchanged after injection, with no changes in the magnitude of G′ and G″ and no rise in the loss tangent. These gels, which may be very lightly chemically crosslinked, due to divinyl contaminants, or physically crosslinked, displayed the most suitable mechanical characteristics as vitreous substitutes. Shear oscillatory tests appear as important criteria in the selection of such materials.     

Intermolecular interactions on amine‐cured epoxy matrices with different crosslink densities. Influence on the hole and specific volumes and the mechanical behavior

The architecture of an epoxy matrix was modified by curing the resin with mono‐/diamine mixtures having identical chemical structures. Both hole volume and specific volume variations were studied by positron annihilation lifetime spectroscopy and pressure‐volume‐temperature/density measurements, respectively. The average hole volume of the networks at room temperature slightly increased when the monoaminic chain extender content increased. The increment in the intermolecular interactions between functional groups of the networks chains, due to the less hindered nitrogen introduced by the monoamine, appears to be the responsible for the observed behavior. Besides, only small variations on the specific volume were observed on increasing the monoamine content, which points out that for a cured epoxy system, the chemical structure of the curing agent is mainly responsible for chain packing in the networks. On the other hand, intermolecular interactions between chains were considered as the key factor for fixing stiffness and strength. Thus, it was observed that the increase of the intermolecular interactions with the monoamine content produced a decrease in the sub‐T_g small‐range cooperative motions, which increased the low‐deformation mechanical properties at temperatures between β and α relaxations. This conclusion could be applied to previous investigations with epoxy matrices not fully crosslinked (nonstoichiometric or noncompletely cured formulations). Finally, it was found that fracture properties do not significantly depend either on the hole volume or on the intermolecular interactions. Fracture properties are more dependent on the crosslink density and the glass transition temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1240–1252, 2009     

Polyepoxide-water interactions: Influence of the chemical structure of the network

We have investigated the influence of the chemical structure of polyepoxide networks on the sorption behaviour and water-induced changes of viscoelastic and mechanical properties. The β relaxation was found to be unaffected by the presence of adsorbed water for an anhydride and an aromatic amine based polyepoxide. The anhydride based network shows better behaviour in a water environment than the aromatic amine based network in terms of plasticisation, reversibility of the degradation and loss of mechanical properties. This has been attributed to (i) a smaller crosslinking density, leading to a smaller specific volume in the glassy state and thus a smaller water uptake and (ii) a strong interaction between water and tertiary amines in epoxide-aromatic amine networks. This interaction, probably of electrostatic nature, would result from the delocalization of the doublet of the nitrogen atom. The results are critically compared to the literature on aliphatic amine based polyepoxides, where the dependence of the β relaxation on the water content is proposed to be related to the electron-donating ability of the aliphatic chain.     

Some composting and biodegradation effects of physically or chemically crosslinked poly(lactic acid)

Polylactide (PLA) crosslinked by using both triallyl isocyanurate (TAIC) and electron radiation or using dicumyl peroxide (DCP) was studied with the aim of examining the behaviour of the modified polymer under various environmental conditions. Thus, the polymer samples were subjected to composting in an industrial pile, exposed to proteinase K, or incubated in sea water. The number-average molecular weight (M_n), melt flow index (MFI), crystallinity (χ), tensile strength (σ_M) and mass loss (in the case of samples treated with proteinase K) were determined. It was found that neat PLA irradiated with high-energy electrons underwent degradation that increased during composting. As a result, the value of M_n of this polymer dramatically decreased. It appeared that PLA crosslinked with TAIC and electron radiation contained, in addition to the crosslinked phase, a phase strongly degraded by this radiation, which facilitated hydrolytic degradation during composting. The σ_M value of PLA crosslinked with TAIC and electron radiation rapidly decreased during composting, whereas that of PLA crosslinked chemically and composted for three weeks slightly increased. As the electron radiation dose increased, the mass loss of PLA containing TAIC and treated with proteinase K decreased, which indicated that the physical crosslinking of PLA hindered enzymatic degradation of this polymer. Important changes in both neat and physically crosslinked PLA incubated in sea water for nine weeks were not detected.     

Epoxy as a reactive plasticizer for improving polycarbonate processibility

An oligomer of a diepoxy (diglycidyl ether of bisphenol-A, DGEBA) and an aromatic diamine (MCDEA) have been used as reactive plasticizers for polycarbonate (PC). A small amount of PC chain scission occurred during this blending process, probably due to transesterification of the PC carbonate group by the hydroxyl group of the DGEBA oligomer. Addition of DGEBA to PC was found to greatly reduce the T_g and processing temperature. Dynamic rheology measurements showed that the added epoxy can very effectively reduce the viscosity, but that the addition of epoxy also accelerated the crystallisation rate of the PC, which was confirmed by XRD, optical transmission microscopy and DMTA. The DMTA results of cured blends also showed that this crystallization of the PC enhanced their heat resistance properties. Sol–gel studies of the cured samples showed that some of the PC was grafted to the crosslinked epoxy network. Studies of the rubbery behaviour, solvent resistance of the cured blend and SEM images suggest that PC is the main continuous phase in the matrix and that the epoxy phase is mainly dispersed as sub-micron particles in the matrix.     

Cure Kinetics of a Tetrafunctional Rubber Modified Epoxy-amine System

In this work the curing kinetics behaviour of a rubber modified epoxy amine system is investigated through calorimetric analysis. This study is part of a wider investigation on new epoxy formulations to be used as matrices of composite materials. The aim is to enhance both the processing behaviour and the mechanical properties of the matrix in order to obtain higher performance composites for more demanding applications. The epoxy system is blended with a high molecular mass rubber containing functional groups reactive towards the epoxies. The formation of a rubber/epoxy network can be achieved by means of a 'pre-reaction' between the epoxy monomers and the rubber functional groups, carried out in the presence of a suitable catalyst and before the resin is cured with the amino hardener. In this work the influence of both the rubber and the catalyst on the resin cure kinetics is analysed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of completely bio‐based epoxy networks derived from epoxidized linseed and castor oil cured with citric acid

Bio‐based epoxy resins were synthesized from nonedible resources like linseed oil and castor oil. Both the oils were epoxidized through in situ method and characterized via Fourier transform infrared and ¹H‐NMR. These epoxidized oils were crosslinked with citric acid without using any catalyst and their properties compared with diglycidyl ether of bisphenol A‐epoxy. The tensile strength and modulus of epoxidized linseed oil (ELO) were found to be more than those of epoxidized castor oil (ECO)‐based network. However, elongation at break of ECO was significantly higher than that of both ELO and epoxy, which reveals its improved flexibility and toughened nature. Thermogravimetric analysis revealed that the thermal degradation of ELO‐based network is similar to that of petro‐based epoxy. Dynamic mechanical analysis revealed moderate storage modulus and broader loss tangent curve of bio‐based epoxies confirming superior damping properties. Bioepoxies exhibit nearly similar contact angle as epoxy and display good chemical resistant. The preparation method does not involve the use of any toxic catalyst and more hazardous solvents, thus being eco‐friendly.     

Synthesis and curing of liquid crystalline epoxy resin based on naphthalene mesogen

Aromatic liquid crystalline epoxy resin (LCE) based on naphthalene mesogen was synthesized and cured with aromatic diamines to prepare heat‐resistant LCE networks. Diaminodiphenylester (DDE) and diaminodiphenylsulfone (DDS) were used as curing agents. The curing reaction and liquid crystalline phase of LCE were monitored, and mechanical and thermal properties of cured LCE network were also investigated. Curing and postcuring peaks were observed in dynamic DSC thermogram. LCE network cured with DDE displayed liquid crystalline phase in the curing temperature range between 183 and 260°C, while that cured with DDS formed one between 182 and 230°C. Glass transition temperature of cured LCE network was above 240°C, and crosslinked network was thermally stable up to 330°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 419–425, 1999     

Development and validation of a novel rheological test device and measurement system for determining the influence of humidity on viscosity

Although a strong impact of humidity on polymers and the formation of their molecular network is known, the complex interactions between humidity, temperature, diffusion, reaction kinetics, rheological behaviour and mechanical properties are not fully understood yet. Most current approaches solely analyse the relationships between environmental conditions in a solidified or crosslinked state and the mechanical properties. In contrast, this work focusses on measuring changes in rheological properties depending on relative humidity in the liquid state. A custom designed novel measuring geometry is introduced and validated. It enables for the first time to rheometrically investigate the influence of relative humidity on reactive systems during their cross-linking process. Our results significantly depict the influence of humidity on the cross-linking process and the gel time. The potential of the introduced measuring geometry for improving reactive systems, adapting these precisely to environmental boundary conditions or assuring product performance is hereby demonstrated. This is especially important for industrial manufacturing processes running at different production sites around the world in different climatic zones or repair processes being performed outdoor, e.g. for wind energy plants.     

Water absorption effects on hydrophilic polymer matrix of carboxyl functionalized glucose resin and epoxy resin

The effects of water absorption on hydrophilic polymer matrices based on carboxylic functionalized glucose (glucose maleic acid ester vinyl resin) and epoxy resins were studied as a function of curing temperature. The matrix cured at higher temperature shows compact crosslinks due to the higher concentration of ether bonds comparing to the matrix cured at the lower temperature. The polymer matrices cured at different temperatures were immersed in water at room temperature for 1000 h and the thermomechanical properties of the cured polymers were characterized using DMA and TGA. Two types of sorbed water were identified. Type I sorbed water contributed mainly to increasing the weight and to the decrease in T_g due to a plasticizer effect. Type II sorbed water was not removed after heating the polymer to 110 °C for an hour. Type II sorbed water causes changes in the mechanical properties of the polymers cured at different temperatures depending on the crosslinks of the matrix. The cured matrix at the higher temperature has the comparatively tight crosslinks in the network structure and the sorbed water molecule disturbed the polymer network resulting in the degradation of the matrix such as microcracks.     

Curing of Styrene-Free Unsaturated Polyester Alkyd and Development of Novel Related Clay Nanocomposites

Thermoset unsaturated polyesters (UP) are usually obtained by crosslinking of alkyd chains dissolved in an unsaturated reactive monomeric diluent, usually styrene. Recently we found that UP-alkyd chains (without styrene) are intrinsically cured into a crosslinked matrix in the presence of peroxide. The thermal, mechanical, dynamic mechanical and chemorheological properties and the network molecular structure of the crosslinked UP-alkyd are a function of the peroxide content used. All properties change considerably upon the addition of small amounts of peroxide (between 1 and 2%wt.) and change to a lesser extent upon employing higher peroxide concentrations (up to 6%wt.). Due to co-occurrence of crosslinking and scission events, the crosslinked system contains both gel and sol fractions. The sol fraction demonstrates a plasticizing effect on the crosslinked network, affecting the thermal and mechanical properties of the crosslinked polymer. The new materials developed in this work are interesting for utilization in innovative styrene-free UP-alkyd/organo-clay nanocomposites. It was found that inducing high shear levels for prolonged durations promotes the intercalation and exfoliation of the silicate layers, resulting in a better dispersion of clay particles. Crosslinking of the UP-alkyd/organo-clay nanocomposites alters their nanostructure, particularly affected by the peroxide content used. Thus, depending on the content employed, either an exfoliated or a combined intercalated/exfoliated structure may be realized.     

Influence of varying hard segments on the properties of chemically crosslinked moisture‐cured polyurethane‐urea

Polyurethane prepolymers are widely used in the reactive hot melt adhesives and moisture‐cured coatings. The chemically crosslinked moisture‐cured formulation based on PEG‐1000 and isophorone diisocyanate was prepared with NCO/OH ratio of 1.6:1.0. Trimethylol propane was used as a crosslinking agent. The excess isocyanate of the prepolymer was chain extended in the ratio of 2:1 (NCO/OH) with different aliphatic diols, and 4:1 with different aromatic diamines. The polymer network maturation during moisture cure was followed by dynamic mechanical thermal analyzer (DMTA) instrument. The thermal and dynamic mechanical properties of the crosslinked polymers were evaluated using thermogravimetric analysis, differential scanning calorimetric analysis and DMTA. Surface properties were evaluated through angle‐resolved X‐ray photoelectron spectroscopy. The present article discusses the physical properties of moisture‐cured polyurethane‐urea (MCPU) containing chemical crosslinks in the hard segment. The complete moisture‐cured polymers showed amorous results toward room temperature modulus, tensile strength, hardness, thermal stability, and transparency. The surface properties showed the enrichment of soft segments. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 102–118, 2006     

Phase behavior,crystallization, and morphology in thermosetting blends of a biodegradable poly(ethylene glycol)‐type epoxy resin and poly(ϵ‐caprolactone)

Thermosetting blends of a biodegradable poly(ethylene glycol)‐type epoxy resin (PEG‐ER) and poly(?‐caprolactone) (PCL) were prepared via an in situ curing reaction of poly(ethylene glycol) diglycidyl ether (PEGDGE) and maleic anhydride (MAH) in the presence of PCL. The miscibility, phase behavior, crystallization, and morphology of these blends were investigated. The uncured PCL/PEGDGE blends were miscible, mainly because of the entropic contribution, as the molecular weight of PEGDGE was very low. The crystallization and melting behavior of both PCL and the poly(ethylene glycol) (PEG) segment of PEGDGE were less affected in the uncured PCL/PEGDGE blends because of the very close glass‐transition temperatures of PCL and PEGDGE. However, the cured PCL/PEG‐ER blends were immiscible and exhibited two separate glass transitions, as revealed by differential scanning calorimetry and dynamic mechanical analysis. There existed two phases in the cured PCL/PEG‐ER blends, that is, a PCL‐rich phase and a PEG‐ER crosslinked phase composed of an MAH‐cured PEGDGE network. The crystallization of PCL was slightly enhanced in the cured blends because of the phase‐separated nature; meanwhile, the PEG segment was highly restricted in the crosslinked network and was noncrystallizable in the cured blends. The phase structure and morphology of the cured PCL/PEG‐ER blends were examined with scanning electron microscopy; a variety of phase morphologies were observed that depended on the blend composition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2833–2843, 2004     

The heterogeneity of the network structure of epoxy polymers studied by dynamic and DSC tests

The final network structure of epoxy polymers cured with a crosslinking agent is strongly connected with the type and amount of the curing agent and conditions. Concerning the morphology of this network, it has been reported that cured epoxies are two-phase systems, containing roughly spherical floccules arranged in a matrix resembling the constituent phases. In this paper and by using an epoxy-polymer cured with different amounts (i.e., different than the stoichiometric one) of a curing agent, strong indications of their network-structure heterogeneity have been disclosed. For this study, two different experimental techniques, such as dynamic mechanical measurements and DSC tests have been utilised. It was found that cured epoxy-polymers consist of two extreme regions, with low and high crosslink densities, whose volume fractions have been estimated by using a mechanical model applicable for two-phase systems.     

Selection,Characterization, and Biodegradation of Surgical Epoxies

A series of epoxy resins has been formulated on the basis of obtaining low water sorption, low water vapor permeability, retention of electrical properties, and resistance to biodegradation by the body. These resins have been tested for these properties both by accelerated aging in 100°C water and in vivo studies.

A literature survey was conducted on the biodegradation of surgical plastics with the findings that nylon lost 80% of its tensile strength after 3 years implantation while Orlon and Dacron deteriorated considerably less in a 2-year period. Teflon, Mastic, and Mylar showed almost no loss in tensile strength after 17 to 22 months.

The epoxies tested on this program showed no loss in strength after 6 months in vivo.

It appears that materials whose chemical structure contain bonds similar to those found in the body (such as amide groups) are susceptible to biodegradation whereas those such as Teflon which contain only C-C bonds or C-F bonds are not.

Two general types of biodegradation can occur on polymers: Attack starting at the end of a polymer chain and proceeding along the chain to produce monomeric fragments (as in polypropylene), and attack at regular intervals along a polymer chain where susceptible cross-linking groups are present to produce macromolecular fragments.

It has been postulated that attack on polymers takes place in the amorphous areas (if they are present) to leave the more crystalline areas of the material intact. Thus, with implantation, these types of materials become brittle.

Histology on the developed epoxies indicated that epoxies containing nonreactive hydrophobic diluents showed a greater foreign body reaction than normal epoxies without such diluents.     

The effect of different hardeners on the ageing of crosslinked epoxy resins

Polymers based on epoxies are widely used in the industrial production of materials for different applications. The crosslinking is often done with hardeners like aliphatic or aromatic amines, with dicarboxylic acid anhydride or anionically. The resins containing mineral fillers possess similar thermal mechanical properties. On thermal oxidative ageing, however, the systems behave very differently. An account is given of the thermal mechanical behaviour and of the weight loss on ageing above the glass transition temperature. By using semiempirical molecular orbital calculations we have been able to identify a possible reason for the different thermal oxidative stability of epoxy resins crosslinked with different hardeners.