Crosslink distribution of epoxy networks studied by small-angle neutron scattering

Crosslink distribution of epoxy networks of diglycidyl ether of bisphenol A (DGEBA) cured with stoichiometric amounts of meta-phenylene diamine (mPDA) was examined by small-angle neutron scattering (SANS). A monodisperse DGEBA resin with the smallest molecular weight was used to enhance the crosslink density and to simplify the network structure for deuterium-labelling. Meta-phenylene-d₄ diamine (mPDAd₄) was applied to label definitively the crosslinks. SANS measurements covered a reciprocal space range from 0.016 to 0.220 Å^?1 or, equivalently, real-space distances from 400 to 30 Å. Application of SANS on the deuterium-labelled epoxy networks consistently produces a constant excess intensity over the unlabelled epoxy networks. Since the scattering intensity from total correlation of the network was negligible, as evident from measurements of SANS on the unlabelled epoxy networks and small-angle X-ray scattering on the epoxy networks, the constant excess SANS intensity can only be attributed to a uniform spatial distribution of the amine curing agent. In other words, the crosslinks are distributed uniformly throughout the epoxy network.     

Preparation and properties of soybean oil–based curing agents for epoxy resin

In order to improve the flexibility properties of conventional epoxy resin, two novel soybean oil–based curing agents were synthesized. The curing agent obtained from the reaction between epoxy soybean oil and ethylene diamine was named EEDA, and another curing agent derived from epoxy soybean oil and isophorone diamine was named EIPDA. Several techniques were used to systematically investigate the effects of the structure and content of the two curing agents on the properties of the cured products. The Fourier transform infrared analysis demonstrated that epoxy resin reacted with soybean oil–based curing agents. The differential scanning calorimetry analysis showed that the curing process between diglycidyl ether of bisphenol‐A (DGEBA) and soybean oil–based curing agents only had an exothermic peak. Thermogravimetric analysis indicated that the cured DGEBA/EIPDA system was more stable than the DGEBA/EEDA system below 300 °C. Mechanical tests and Shore D hardness tests suggested that excessive EEDA greatly enhanced the toughness of cured products because of the introduction of aliphatic chains.© 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44754.     

A comparison of chemorheological models for thermoset cure

A chemorheological study of a thermoset system consisting of diglycidyl ether of bisphenol-A (DGEBA) epoxy resin and a mixture of two armoatic amine curing agents; 4,4′ methylenedianiline (MDA) and m-phenylene diamine (m-PDA), was conducted. Experimentally obtained viscosity data were checked against the predictions of two different viscosity models; one based on a phenomenological equation obtained by modification of the classical Williams–Landel–Ferry (WLF) equation and the other based on an extension of the branching theory originally proposed by Flory. In general, the predictions of both models were in excellent agreement with experimentally obtained isothermal and dynamic viscosity data. The branching theory model was found to have a slight advantage over the phenomenological equation model in describing the viscosity prior to gelation in a fast heating cure cycle.     

Kinetic study of polymerization/curing of filament-wound composite epoxy resin sytems with aromatic diamines

The kinetics of the polymerization/curing of an 80/20 blend of a diglycidyl ether of bisphenol A (DGEBA) and a diglycidyl ether of 1,4-butanediol (DGEBD) with a commercial mixture of methylene dianiline and m-phenylene diamine (Tonox 60/40) was studied, at amine/epoxy ratios ranging from 1.1 to 4.4. Fourier transform infrared (FT-IR) measurements were used to follow the extent of epoxy conversion at 18–122°C, and bulk viscosity measurements were used to define the working range of the resin. For an amine/epoxy ratio of 1.1, the activation energy for the polymerization/curing reaction (based on time to 50% epoxy conversion in the S-shaped conversion-time plots) was 11.9 kcal/mole by FT-IR. This value compared favorably with the corresponding value of 12.7 kcal/mole obtained by Moroni and co-workers in a complementary differential scanning calorimetry (DSC) study of the same system. The FT-IR conversion-time plots were fitted to a kinetic expression that can be accommodated by an autocatalytic mechanism; the expression contains two rate constants with activation energies of 13.7 and 10.0 kcal/mole, respectively. The viscosity of the curing epoxy resin was found to obey the Williams-Landel-Ferry equation, with a Di Benedetto expression for the glass transition temperature.     

Aromatic phosphorodiamidate curing agent for epoxy resin coating with flame-retarding properties

Two organophosphorus-based diamines containing aromatic moieties has been synthesized and used as a curing and flame retarding agent for epoxy resin coatings. This agent functions not only as a crosslinking materials in the Epon 828 epoxy resin curing process but also as a fire retarding compound to produce thin films or composites upon curing. Phenyl phosphonic ethylene diamine diamide (PPEDD) was synthesized via condensation of phenylphosphonic dichloride (PPDC and ethylenediamine (EDA)). Likewise, phenyl phosphonic p-phenylene diamine diamide (PPPDD) was synthesized via condensation of PPDC and p-phenylenediamine (PDA). Kinetics studies of the curing reaction of the two phosphorodiamidates were carried out in comparison with the corresponding non-phosphorus containing reference crosslinking agents, EDA and PDA. Thermal stability of the cured epoxy were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Moreover, flame retardant properties of the materials was investigated by limiting oxygen index (LOI) measurements. The results show that epoxy resin cured with phosphorodiamidate possesses higher thermostability than that of the non-phosphorus containing counterpart. This is evident by a significantly higher amount of char formed upon burning. More importantly, the LOI of 27 and 31 was observed in the PPEDD-cured epoxy resin and PPPDD-cured epoxy resin compared with those prepared from non-phosphorus curing agents (20 for EDA and 21 for PDA). This was obtained only with approximately 2–3 wt.% of phosphorus content.     

Synthesis and characterization of diphenylsilanediol modified epoxy resin and curing agent

A series of diphenylsilanediol modified epoxy resins and novel curing agents were synthesized. The modified epoxy resins were cured with regular curing agent diethylenetriamine (DETA); the curing agents were applied to cure unmodified diglycidyl ether of bisphenol A epoxy resin (DGEBA). The heat resistance, mechanical property, and toughness of all the curing products were investigated. The results showed that the application of modified resin and newly synthesized curing agents leads to curing products with lower thermal decomposition rate and only slightly decreased glass transition temperature (T_g), as well as improved tensile modulus and tensile strength. In particular, products cured with newly synthesized curing agents showed higher corresponding temperature to the maximum thermal decomposition rate, comparing with products of DGEBA cured by DETA. Scanning electron microscopy micro images proved that a ductile fracture happened on the cross sections of curing products obtained from modified epoxy resins and newly synthesized curing agents, indicating an effective toughening effect of silicon–oxygen bond.     

Synthesis and characterization of chlorinated soy oil based epoxy resin/glass fiber composites

Composites with good toughness properties were prepared from chemically modified soy epoxy resin and glass fiber without additional petroleum based toughening agent. Chlorinated soy epoxy (CSE) resin was prepared from soybean oil. The CSE was characterised by spectral, and titration method. The prepared CSE was blended with commercial epoxy resin in different ratios and cured at 85°C for 3 h, and post cured at 225°C for 2 h using m‐phenylene diamine (MPDA) as curing agent. The cure temperatures of epoxy/CSE/MPDA with different compositions were found to be in the range of (151.2–187.5°C). The composite laminates were fabricated using epoxy /CSE/MPDA‐glass fiber at different compositions. The mechanical properties such as tensile strength (248–299 MPa), tensile modulus (2.4–3.4 GPa), flexural strength (346–379 MPa), flexural modulus (6.3–7.8 GPa) and impact strength (29.7–34.2) were determined. The impact strength increased with the increase in the CSE content. The interlaminor fracture toughness (G_IC) values also increased from 0.6953 KJ/m² for neat epoxy resin to 0.9514 KJ/m² for 15%CSE epoxy‐modified system. Thermogravimetric studies reveal that the thermal stability of the neat epoxy resin was decreased by incorporation of CSE. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Layered silicate nanocomposites based on various high‐functionality epoxy resins: The influence of an organoclay on resin cure

The influence of an organically modified clay on the curing behavior of three epoxy systems widely used in the aerospace industry and of different structures and functionalities was studied. Diglycidyl ether of bisphenol A (DGEBA), triglycidyl p‐amino phenol (TGAP) and tetraglycidyl diamino diphenylmethane (TGDDM) were mixed with an octadecyl ammonium ion modified organoclay and cured with diethyltoluene diamine (DETDA). The techniques of dynamic mechanical thermal analysis (DMTA), chemorheology and differential scanning calorimetry (DSC) were applied to investigate gelation and vitrification behavior, as well as catalytic effects of the clay on resin cure. While the formation of layered silicate nanocomposite based on the bifunctional DGEBA resin has been previously investigated to some extent, this paper represents the first detailed study of the cure behavior of different high performance, epoxy nanocomposite systems.     

Core-shell particles designed for toughening the epoxy resins. II. Core-shell-particle-toughened epoxy resins

The diglycidyl ether of bisphenol A–m-phenylene diamine (DGEBA–MPDA) epoxy resin was toughened with various sizes and amounts of reactive core-shell particles (CSP) with butyl acrylate (BA) as a core and methyl methacrylate (MMA) copolymerized with various concentration of glycidyl methacrylate (GMA) as a shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either core or shell. Among the variables of incorporated CSP indicated above, the optimal design was to obtain the maximum plastic flow of epoxy matrix surrounding the cavitated CSP during the fracture test. It could be achieved by maximizing the content of GMA in a shell-crosslinked CSP, the particle size, and the content of CSP in the epoxy resin without causing the large-scale coagulations. The incorporation of reactive CSP could also accelerate the curing reaction of epoxy resins. Besides, it was able to increase the glass transition temperature of epoxy resins if the particle size ≤0.25 μm and the dispersion was globally uniform. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2313–2322, 1998     

Curing and Morphology of BPA Epoxy Resin/LC Epoxy Resin PEPEB Composites

Liquid crystalline epoxy resin (LC epoxy resin) – p-phenylene di{4-[2-(2,3-epoxypropyl)ethoxy]benzoate} (PEPEB) was synthesized. The mixture of PEPEB with bisphenol-A epoxy resin (BPAER) was cured with a curing agent 4,4-diamino-diphenylmethane (DDM). The curing process and thermal behavior of this system were investigated by differential scanning calorimeter (DSC) and torsional braid analysis (TBA). The morphological structure was measured by polarizing optical microscope (POM) and scanning electron microscope (SEM). The results show that the initial curing temperature Ticu (gel point) of this system is 68.1°C, curing peak temperature T _pcu is 102.5°C, and the disposal temperature T _fcu is 177.6°C. LC structure was fixed in the cured epoxy resin system. The curing kinetics was investigated by dynamic DSC. Results showed that the curing reaction activation energy of BEPEB/BPAER/DDM system is 22.413 kJ/mol. The impact strength is increased 2.3 times, and temperature of mechanical loss peak is increased to 23°C than the common bisphenol-A epoxy resin, when the weight ratio of BEPEB with BPAER is 6 100.     

Curing kinetics and properties of epoxy resin-fluorenyl diamine systems

Diglycidyl ether of bisphenol fluorene (DGEBF), 9,9-bis-(4-aminophenyl)-fluorene (BPF) and 9,9-bis-(3-methyl-4-aminophenyl)-fluorene (BMAPF) were synthesized to introduce more aromatic structures into the epoxy systems, and their chemical structures were characterized with FTIR, NMR and MS analyses. The curing kinetics of fluorenyl diamines with different epoxy resins including DGEBF, cycloaliphatic epoxy resin (TDE-85) and diglycidyl ether of bisphenol A (DGEBA) was investigated using non-isothermal differential scanning calorimetry (DSC), and determined by Kissinger, Ozawa and Crane methods. The thermal properties of obtained polymers were evaluated with dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results show that the values of activation energy (E_a) are strongly dependent on the structures of epoxy resin and curing agent. The curing reactivity of epoxy system is restrained by the introduction of rigid fluorene into chain backbone and flexible methyl into side groups. The cured DGEBF/fluorenyl diamine systems exhibit remarkably higher glass transition temperature, better thermal stability and lower moisture absorption compared to those of DGEBA/fluorenyl diamine systems, and display approximate heat resistance and much better moisture resistance relative to those of TDE-85/fluorenyl diamine systems.     

Effect of reactive poly(ethylene glycol) flexible chains on curing kinetics and impact properties of bisphenol‐a glycidyl ether epoxy

Bisphenol‐A glycidyl ether epoxy resin was modified using reactive poly(ethylene glycol) (PEO). Dynamic mechanical analysis showed that introducing PEO chains into the structure of the epoxy resin increased the mobility of the molecular segments of the epoxy network. Impact strength was improved with the addition of PEO at both room (RT) and cryogenic (CT, 77 K) temperature. The curing kinetics of the modified epoxy resin with polyoxypropylene diamines was examined by differential scanning calorimetry (DSC). Curing kinetic parameters were determined from nonisothermal DSC curves. Kinetic analysis suggested that the two‐parameter autocatalytic model suitably describes the kinetics of the curing reaction. Increasing the reactive PEO content decreased the heat flow of curing with little effect on activation energy (E_a), pre‐exponential factor (A), or reaction order (m and n). © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Tetraglycidyl epoxy resins and graphite fiber composites cured with flexibilized aromatic diamines

Studies were performed to synthesize new ether modified, flexibilized aromatic diamine hardeners for curing epoxy resins. The effect of moisture absorption on the glass transition temperatures of a tetraglycidyl epoxy, MY 720, cured with flexibilized hardeners and a conventional aromatic diamine was studied. Unidirectional composites, using epoxy-sized Celion 6000 graphite fiber as the reinforcement, were fabricated. The room temperature and 300°F mechanical properties of the composites, before and after moisture exposure, were determined. The Mode I interlaminar fracture toughness of the composites was characterized, using a double cantilever beam technique to calculate the critical strain energy release rate, G_IC.     

Synthesis of curing agent for epoxy resin based on halogenophosphazene

Epoxy resins are, due to their excellent properties (such as chemical resistance, dimensional stability, and heat resistence), widely used in practice. The basic principle of curing epoxy resins with a hardener containing multiple amino groups is the crosslinking reaction between active hydrogen atoms in the hardener and the oxirane groups in the epoxy resin. This study deals with the synthesis and characterization of hexachloro‐cyclo‐triphosphazene derivative and its subsequent use for curing epoxy resins. The new hardener was prepared from hexachloro‐cyclo‐triphosphazene by nucleophilic substitution with isophorone diamine and its curing capability was compared with original isophorone diamine. The prepared derivative hexaisophorone diamino‐cyclo‐triphosphazene (HICTP) provided advantages over conventional curing system, as it improved mechanical properties as well as the flame resistance. Testing of the cured epoxy resin during burning was carried out using dual cone calorimeter, which enables more extensive monitoring of parameters in comparison with testing using oxygen index that has been used in many publications. The epoxy resin cured with the prepared phosphorus containing HICTP exhibits lower values for total heat release, amount of smoke released and oxygen consumed, which may cause a slower flame spread. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42917.     

Toughening of Diglycidyl Ether of Bisphenol-A Epoxy Resin Using Poly (Ether Ether Ketone) with Pendent Ditert-Butyl Groups

Poly(ether ether ketone) (PEEKDT), hydroxyl terminated poly(ether ether ketone) (PEEKDTOH) and fluorine terminated poly (ether ether ketone) (PEEKDTF) with pendent ditert-butyl groups were synthesized by the nucleophilic substitution reaction of 4,4′-difluorobenzophenone with 2,5-ditert-butylhydroquinone in N-methyl-2-pyrrolidone medium using anhydrous potassium carbonate as catalyst. Diglycidyl ether of bisphenol-A epoxy resin was blended with PEEKDT, PEEKDTOH, and PEEKDTF, and cured with 4,4′-diaminodiphenylsulfone (DDS). The polymers formed heterogeneous blends before curing, and upon curing the polymers got dispersed in the epoxy matrix. The mechanical properties of the cured blends were slightly lower than that of the unmodified resin. The fracture toughness increased with the addition of ditert-butyl PEEK into epoxy resin and the extent of improvement was dependent on the type of modifier used. Hydroxyl terminated polymers gave up to 40% increase in fracture toughness. The dynamic mechanical spectrum of the blends showed only a single T_g due to the proximity of the glass transition temperature of modified PEEK and DDS cured epoxy resin.     

Multifunctional epoxy resins

The curing behavior of epoxy resins prepared by reacting epichlorohydrin with 4,4′-diaminodiphenyl methane (DADPM)/4,4′-diaminodiphenyl ether (DADPE) or 4,4′-diaminodiphenyl sulfone (DDS) was investigated using DDS and tris-(m-aminophenyl)phosphine oxide (TAP) as curing agents. A broad exothermic transition with two maxima were observed in the temperature range of 100–315°C when TAP was used as the curing agent. The effect of varying DDS concentration on curing behavior of epoxy resin was also investigated. Peak exotherm temperature (T_exo) decreased with increasing concentration of DDS, whereas heat of curing (ΔH) increased with an increase in amine concentration up to an optimum value and then decreased. Thermal stability of the resins, cured isothermally at 200°C for 3 h, was investigated using thermogravimetric analysis in a nitrogen atmosphere. Glass fiber-reinforced multifunctional epoxy resin laminates were fabricated and the mechanical properties were evaluated. © 1993 John Wiley & Sons, Inc.     

Synthesis and properties of trifluoromethyl groups containing epoxy resins cured with amine for low Dk materials

The study synthesized a trifluoromethyl (CF₃) groups with a modified epoxy resin, diglycidyl ether of bisphenol F (DGEBF), using environmental friendly methods. The epoxy resin was cured with 4,4′‐diaminodiphenyl‐methane (DDM). For comparison, this study also investigated curing of commercially available diglycidyl ether of bisphenol A (DGEBA) with the same curing agent by varying the ratios of DGEBF. The structure and physical properties of the epoxy resins were characterized to investigate the effect of injecting fluorinated groups into epoxy resin structures. Regarding the thermal behaviors of the specimens, the glass transition temperatures (T_g) of 50–160°C and the thermal decomposition temperatures of 200–350 °C at 5% weight loss (T_d5%) in nitrogen decreased as amount of DGEBF increased. The different ratios of cured epoxy resins showed reduced dielectric constants (D_k) (2.03–3.80 at 1 MHz) that were lower than those of pure DGEBA epoxy resins. Reduced dielectric constant is related to high electrronegativity and large free volume of fluorine atoms. In the presence of hydrophobic CF₃ groups, the epoxy resins exhibited low moisture absorption and higher contact angles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Frontal curing of epoxy resins: Comparison of mechanical and thermal properties to batch-cured materials

The epoxy resin diglycidyl ether of bisphenol F (DGEBF) was cured by the aliphatic amine curing agent Epicure 3371 in a stoichiometric ratio both frontally and in a batch-cure schedule. Glass transition temperatures (T_g) were determined using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). DMA also was used for studying the storage modulus (E′) and tan delta (tan δ) of the cured samples. Tensile properties of epoxy samples were tested according to ASTM D638M-93. The properties of the frontally cured epoxy resin were found to be very close to that of batch-cured epoxy resin. Velocity of cure-front propagation was measured for both neat and filled epoxy. Rubber particles (ground tires) were used as a filler. The maximum percentage of filler in the epoxy resin allowing propagation was 30%. Because of convection, only descending fronts would propagate. Advantages and disadvantages of frontal curing of epoxy resins are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1209–1216, 1997     

Determination of dielectric and calorimetric properties in the cure reaction of two thermosets by dielectric analysis and differential scanning calorimetry

Dielectric analysis and differential scanning calorimetry were used to study the cure reaction of an epoxy resin (the diglycidyl ether of bisphenol A (DGEBA) (n = 0)) and two curing agents, ie 1,2‐diamine cyclohexane (1,2‐DCH) and m‐xylylenediamine (m‐XDA). Various dielectric properties, such as the relaxed permittivity, unrelaxed permittivity and dipole strength, were determined as a function of the curing time, temperature and conversion. The dielectric curing properties observed were related to geometric and energetic aspects of the samples. Analysis of the results shows that the relaxed permittivity and dipole strength are higher for the system DGEBA (n = 0)/1,2‐DCH than for the system DGEBA (n = 0)/m‐XDA. Copyright © 2005 Society of Chemical Industry