Toughening of epoxy systems by brominated epoxy

Blends of brominated epoxy (BE) and conventional epoxy resins were studied following curing with aliphatic triethylenetetramine (TETA), etheric (polyether diamine‐ PEA4), and aromatic (3,3′‐diamino diphenyl sulfone [DDS]) hardeners. The addition of BE resulted in an increase in T_g in all tested blends. Blends with 50 wt% BE cured with TETA demonstrated an increase in flexural modulus and flexural strength, while preserving the elongation. Blends with 40 wt% BE cured with PEA4 and 50 wt% BE cured with DDS resulted in a significant enhanced tensile elongation. The shear strength of all cured systems decreased moderately with the addition of BE exhibiting a mixed mode failure. Analysis of the fracture morphology using electron microscopy supported the increase of toughness levels as a result of incorporating BE to conventional epoxy. A unique nodular and rough fracture morphology was obtained, which is related to a toughening mechanism caused by the addition of BE. It was concluded that blends of BE and conventional epoxy could be used as structural adhesives having high T_g, enhanced mechanical properties and increased toughness. POLYM. ENG. SCI., 59:206–215, 2019. © 2018 Society of Plastics Engineers     

Bisphenol containing novel polyimides/glass fiber composites

A new diamine was synthesized using bisphenol‐A and p‐amino benzoic acid. Polyimides I and II were prepared with the diamine and pyromellitic dianhydride/3,3′,4,4′ benzophenone tetracarboxylic acid dianhydride. Bismaleimide (BMI) was synthesized using the same diamine and maleic anhydride. The prepared diamine and polyimides were characterized using FTIR. Thermo gravimetric analysis was used to study the thermal properties of synthesized polyimides and BMI. Woven glass fabric/unidirectional glass fiber‐polyimide/BMI composites were made and their properties (fiber volume fraction, density, tensile, flexural, impact, and hardness) were studied and compared with a few representative carbon fiber polyimide, carbon fiber–epoxy, and glass fiber–epoxy composites. The prepared composites were subjected to thermal aging and moisture absorption and their effects on tensile and flexural properties were studied. POLYM. COMPOS., 28: 372–380, 2007. © 2007 Society of Plastics Engineers     

马来海松酸类环氧树脂固化产物性能

本文研究了马来海松酸类环氧树脂与酸酐及芳香二胺类五种固化剂固化产物的耐热性能和机械性能,并详细讨论了固化后树脂的性能与环氧树脂、固化剂的化学结构及固化反应条件之间的关系。     

Synthesis and curing of new epoxycycloaliphatic polyesterimides with dianhydrides and diisocyanates as hardeners

New epoxy resins obtained from a series of bis(4,5-epoxytetrahydrophthalimides) and three different dicarboxylic diacids were prepared and characterized by spectroscopic techniques and thermal analyses. Homopolymerization of epoxy groups was observed as a parallel process to the polycondensation reaction. Using Epiclon B with a tertiary amine and hexamethylene diisocyanate as hardeners, we studied the reaction of epoxy groups and hydroxylic groups attached to the cycloaliphatic moiety in the main chain. No exotherms were clearly detected by DSC, and during the curing process, only increases in the T_g, values were observed. These cycloaliphatic epoxy polyesters have similar thermal characteristics to related aromatic epoxy polyesterimides, but have better processability. © 1996 John Wiley & Sons, Inc.     

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.     

Effects of chemical modification of epoxy‐(ethylene diamine) on water hyacinth fibers (Eichhornia crassiper)‐filled (low‐density polyethylene)/(natural rubber) blends

The effects of modification by epoxy‐(ethylene diamine) (EED) on (water hyacinth fiber)‐filled (low‐density polyethylene)/(natural rubber) (LDPE/NR/WHF_‐EED) composites were studied. The LDPE/NR/WHF and LDPE/NR/WHF_‐EED composites were prepared by using a Brabender Plasticorder. LDPE/NR/WHF_‐EED showed higher tensile strength, Young's modulus, and elongation at break but lower molar sorption of toluene compared with LDPE/NR/WHF composites. The Fourier‐transform infrared radiation analysis indicated that the absorption peak at 1,648 cm^?1 exhibited the C? N band. This C? N band was formed from the bond of the epoxy group and the amine group in the WHF‐g‐epoxy‐(ethylene diamine) (WHF_‐EED). The scanning electron microscopic morphology of LDPE/NR/WHF_‐EED composites displayed rougher surfaces and less fiber pull, which improved the interfacial adhesion among the fiber/matrix. J. VINYL ADDIT. TECHNOL., 20:201–209, 2014. © 2014 Society of Plastics Engineers     

Novel phosphorus‐containing hardeners with tailored chemical structures for epoxy resins: Synthesis and cured resin properties

A comparative evaluation of systematically tailored chemical structures of various phosphorus‐containing aminic hardeners for epoxy resins was carried out. In particular, the effect of the oxidation state of the phosphorus in the hardener molecule on the curing behavior, the mechanical, thermomechanical, and hot‐wet properties of a cured bifunctional bisphenol‐A based thermoset is discussed. Particular attention is paid to the comparative pyrolysis of neat cured epoxy resins containing phosphine oxide, phosphinate, phosphonate, and phosphate (with a phosphorus content of about 2.6 wt %) and of the fire behavior of their corresponding carbon fiber‐reinforced composites. Comparatively faster curing thermosetting system with an enhanced flame retardancy and adequate processing behavior can be formulated by taking advantage of the higher reactivity of the phosphorus‐modified hardeners. For example, a combination of the high reactivity and of induced secondary crosslinking reactions leads to a comparatively high T_g when curing the epoxy using a substoichiometric amount of the phosphinate‐based hardener. The overall mechanical performance of the materials cured with the phosphorus‐containing hardeners is comparable to that of a 4,4′‐DDS‐cured reference system. While the various phosphorus‐containing hardeners in general provide the epoxy‐based matrix with enhanced flame retardancy properties, it is the flame inhibition in the gas phase especially that determines the improvement in fire retardancy of carbon fiber‐reinforced composites. In summary, the present study provides an important contribution towards developing a better understanding of the potential use of such phosphorus‐containing compounds to provide the composite matrix with sufficient flame retardancy while simultaneously maintaining its overall mechanical performance on a suitable level. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Subtle variations in the structure of crosslinked epoxy networks and the impact upon mechanical and thermal properties

Presented here is an investigation of the structure–property relationships of crosslinked networks using three bi-functional glycidyl ether aromatic epoxy resins, two bi-aryl and one tri-aryl, cured with bi- and tri-aryl amines. Subtle changes to the monomer chemistry including changing aromatic substitution patterns from meta to para, methylene to isopropyl and isopropyl to ether were explored. Changing an epoxy resin backbone from methylene to isopropyl enhances backbone rigidity thus increasing glass transition temperature (T_g), yield strength, and strain despite reducing modulus. Changing meta-substitution to para increases T_g and yield strain while leaving strength unaffected and reducing modulus. Changing isopropyl linkages to ether reduces modulus, strength, T_g, and yield strain reflecting increased molecular flexibility. Using three instead of two aromatic rings increases the molecular weight between crosslinks thereby decreasing T_g and yield strain while increasing modulus and strength. Despite the complexities of multiple systems for varying epoxy resins and amine hardeners, the effect upon network properties is explained in terms of short- and long-range molecular and segmental mobility, crosslink density, and equilibrium packing density. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48874.     

Epoxy/imidazolium-based ionic liquid systems: The effect of the hardener on the curing behavior,thermal stability,and microwave absorbing properties

A new transparent microwave absorbing coating was developed by compounding 1-butyl-3-methyl imidazolium tetrafluoroborate (bmim.BF₄) ionic liquid (IL) with diglycidyl ether of bisphenol A-type epoxy resin. The systems were crosslinked with the IL alone or combined with conventional hardeners, as anhydride or aromatic amine. The curing behavior was investigated by thermal and spectroscopic analysis performed at high temperatures. Neat bmim.BF₄ was able to cure epoxy resin, giving rise to networks with outstanding thermal stability compared with the systems cured with anhydride or aromatic amine. bmim.BF₄ accelerated the curing process in the presence of aromatic amine but retarded this event when anhydride was used as an external curing agent. The glass-transition temperature evaluated by dynamic mechanical analysis decreased when the amount of IL increased, which can be attributed to side reactions during the curing process, as well as the plasticizing effect of IL. The epoxy networks cured with bmim.BF₄ alone or in combination with anhydride or aromatic amine were transparent and presented considerable microwave absorbing properties in the X-band frequency range (8–12 GHz), being the best performance observed for the systems cured with bmim.BF4/anhydride curing system, with reflection loss value around −16 dB at 11.3 GHz. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48326.     

Moisture absorption phenomenon in permeable fiber polymer composites

Studies were conducted on the moisture absorption characteristics of jute fiber composites based on polyester and epoxy resin systems, under constant humidity (ø) and ambient temperature (T) conditions. The initial slope of the moisture absorption curve (a direct measure of the composite diffusivity) increased with increased superficial fiber volume fraction (V_f), where as the time (t_m'), needed to reach the equilibrium moisture absorption value showed a reversed trend. This behavior is just a reverse to that observed¹ in case of composites with practically impermeable fibers (e.g., glass and graphite) in the same resin matrices. The theoretical expressions governing moisture diffusion phenomenon in impermeable fiber composites were modified and analyzed for the case of composites containing a permeable fiber. The experimental data obtained on the latter were then discussed in relation to the modified theory. The meaning of a correct fiber volume fraction (V_f,) as applicable to permeable fiber composites was defined.     

Time-temperature–transformation (TTT) diagrams of high Tg epoxy systems: Competition between cure and thermal degradation

The cure behavior and thermal degradation of high T_g epoxy systems have been investigated by comparing their isothermal time-temperature-transformation (TTT) diagrams. The formulations were prepared from di- and trifunctional epoxy resins, and their mixtures, with stoichiometric amounts of a tetrafunctional aromatic diamine. The maximum glass transition temperatures (T_g∞) were 229°C and > 324°C for the fully cured di- and trifunctional epoxy materials, respectively. Increasing functionality of the reactants decreases the times to gelation and to vitrification, and increases the difference between T_g after prolonged isothermal cure and the temperature of cure. At high temperatures, there is competition between cure and thermal degradation. The latter was characterized by two main processes which involved devitrification (decrease of modulus and T_g) and revitrification (char formation). The experimentally inaccessible T_g∞ (352°C) for the trifunctional epoxy material was obtained by extrapolation from the values of T_g∞ of the less highly crosslinked systems using a relationship between the glass transition temperature, crosslink density, and chemical structure.     

Synthesis of a Sustainable and Bisphenol A-Free Epoxy Resin Based on Sorbic Acid and Characterization of the Cured Thermoset

In the present study, an epoxy compound, 1,2-epoxy-6-methyl-triglycidyl-3,4,5-cyclohexanetricarboxylate (EGCHC) synthesized from sorbic acid, maleic anhydride, and allyl alcohol is proposed. Using commodity chemicals, a bio-based carbon content of 68.4 % for the EGCHC resin is achieved. When cured with amine hardeners, the high oxirane content of EGCHC forms stiff cross-linked networks with strong mechanical and thermal properties. The characterization of the epoxy specimens showed that EGCHC can compete with conventional epoxy resins such as DGEBA. A maximum stiffness of 3965 MPa, tensile strength of 76 MPa, and T_g of 130 °C can be obtained by curing EGCHC with isophorone diamine (IPD). The cured resin showed to be decomposable under mild conditions due to the ester bonds. The solid material properties of EGCHC expose its potential as a promising bisphenol A, and epichlorohydrine free alternative to conventional petroleum-based epoxies with an overall high bio-based carbon content.     

Synthesis,Morphology and Viscoelastic Properties of Epoxy/Polyhedral Oligomeric Silsesquioxane (POSS) and Epoxy/Cyanate Ester/POSS Nanocomposites

TriSilanolPhenyl-polyhedral oligomeric silsesquioxane (POSS-1) (C₄₂H₃₈O₁₂Si₇), 1–15 wt%, was incorporated into aliphatic epoxy resin (Clearstrem Products, Inc.) with aliphatic diamine curing agents and cured. This epoxy resin was also blended with an equal weight (50/50 w/w) of aromatic cyanate ester resin, Lonza’s PT-15, and 1–15 wt% of POSS-1 and cured. These composites were characterized by FT-IR, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (X-EDS), dynamic mechanical thermal analysis (DMTA) and three-point bending flexural tests. XRD and X-EDS measurements were consistant with partial molecular dispersion of the POSS units in the continuous matrix phase, together with POSS aggregates. TEM and SEM show that POSS-1-enriched nanoparticles are present in the matrix resins of both the epoxy/POSS and epoxy/cyanate ester/POSS-1 composites. The storage bending moduli, E′, in the rubbery region and the glass transition temperatures, T_g, of epoxy and epoxy/cyanate ester 1-5% POSS-1 composites are higher than those of the reference resins. Small amounts (≤5 wt%) of POSS-1 improved E′ and T_g of both systems and raised flexural strengths and moduli.     

Synthesis and curing of rod-like epoxies with alkoxy groups as flexible side chain

Summary A series of aromatic diepoxides consisting of rigid ester unit with different lengths of flexible alkoxy side groups (R = H, OCH₃, OC₃H₇, and OC₈H₁₇) were synthesized, and their thermal and cure behavior with aromatic diamine were investigated by using a DSC. The values of melting temperatures for the epoxy monomers ranged from 65 to 221°C depending upon the length of side groups. Cure kinetics of an epoxy/4,4′-diaminodiphenylmethane (DDM) system with two different side groups (R = OC₃H₇ and OC₈H₁₇) forming homogeneous mixture was examined using a multi-temperature scan method, developed by Ozawa and Kissinger, in order to determine the activation energy (E) and the frequency factor (A). The epoxy monomer having longer side group showed smaller E and larger A, indicating that the introduction of the long flexible side group could accelerate the cure reaction of rod-like epoxy. Received: 15 January 2002 / Accepted: 4 March 2002     

Epoxy/oil fly ash composites prepared through in situ polymerization: Enhancement of thermal and mechanical properties

The utilization of oil fly ash (OFA) as a filler in polymer composites to enhance their strength and flow properties and reduce the cost of fabrication is a promising technique. OFA filled epoxy composites, based on bisphenol‐A liquid epoxy, were prepared via in situ co‐polymerization with isophorone diamine as a curing agent. In the present work, the possibility of using residual OFA (<30 μm) as filler in epoxy composites was studied using thermal, mechanical, and morphological characterization techniques. The results showed a significant improvement in the performance of epoxy composites containing OFA. In addition, OFA filled epoxy exhibited a higher resistance to degradation in acidic and basic environments when compared to unfilled epoxy. Statistical analysis tools were used to determine the significance of the improvements. It is proposed that up to 4 wt% of OFA can be used in epoxy industrial pipes to improve their corrosive chemical resistance properties without affecting their bulk physical properties. POLYM. COMPOS., 37:512–522, 2016. © 2014 Society of Plastics Engineers     

Effects of hygrothermal aging on the thermomechanical properties of a carbon fiber reinforced epoxy sheet molding compound: An experimental research

Carbon fiber sheet molding compounds (C-SMCs) are discontinuous fiber reinforced composite materials. Among them, epoxy-based C-SMCs are becoming relevant materials due to their high thermomechanical performance and better formability than continuous fiber reinforced composites. The thermomechanical performance of epoxy resins and epoxy based continuous carbon fiber composites have shown to be influenced by hygrothermal aging. In this work, this influence is studied for an epoxy-based C-SMC. Epoxy-based C-SMC samples were hygrothermally aged by means of accelerated conditioning, exposing them to 65% relative humidity, and 80°C in a climatic chamber. The equilibrium moisture content, as well as the moisture diffusion coefficient has been determined. The thermomechanical properties of epoxy C-SMC have been analyzed by dynamic mechanical analysis, tensile, 3-point bending, and short beam tests in dry and aged samples. The results showed that epoxy C-SMC is affected by hygrothermal aging in the cases of moisture intake and its effects on T_g value, but interestingly, the hygrothermal aging did not generate any degradation effects in the mechanical response of epoxy C-SMC.     

Epoxy matrices for filament-wound carbon fiber composites

Epoxies suitable for filament-winding fibrous composites must be processible at ambient temperatures, nontoxic, chemically simple, undergo full cure at ≤ 100°C and, also, be tough and exhibit a T_g > 120°C. In this paper, we report the cure characteristics, processibility, toxicity, and mechnical and physical properties of a number of amine-cured diglycidyl ether of bisphenol-A (DGEBA) epoxide candidate systems suitable for filament-wound carbon fiber composites. 2,5-Dimethyl-2,5-hexane diamine (DMHDA)-cured DGEBA epoxy was found to be the most promising candidate. The good processibility and thermal properties, together with the low cure characteristics of the DGEBA–DMHDA epoxy system, are discussed in terms of molecular structure of the amine molecule. The network structural parameters that control epoxy toughness and subsequent embrittlement upon plastic flow are discussed. Evidence is presented for plastic flow-induced thermal and mechanical property deterioration of epoxies as a result of network chain scission.     

Surfactant-Assisted Dispersion of MWCNTs in Epoxy Resin Used in CFRP Strengthening Systems

The epoxy resin used as the bonding agent in carbon fiber-reinforced polymer (CFRP) strengthening systems was modified by the infusion of multiwalled carbon nanotubes (MWCNTs). Two types of surfactants, Triton X-100 and C₁₂E₈, were used to disperse the nanotubes in the epoxy resin employing ultrasonic mixing. Dynamic mechanical analysis and tensile tests were conducted to study the effect of the surfactant-assisted dispersion of nanotubes on the thermal and mechanical properties of epoxy composites. The morphology of the epoxy composites was interpreted using scanning electron microscopy (SEM). Moreover, the effect of surfactant treatment on the structure of nanotubes was investigated by Fourier transform infrared (FT-IR). Based on the experimental results, the tensile strength and the storage modulus of the epoxy resin were increased by 32% and 26%, respectively, by the addition of MWCNTs. This was attributed to the homogeneous dispersion of nanotubes in the epoxy resin according to the SEM images. Another reason for the enhancement in the tensile properties was the reinforced nanotube/epoxy interaction as a result of the surfactant anchoring effect which was proved by FT-IR. A moderate improvement in the glass transition temperature (T _g) was recorded for the composite fabricated using Triton X-100, which was due to the restricted molecular motions in the epoxy matrix. To characterize the temperature-dependent tensile behavior of the modified epoxy composites, tensile tests were conducted at elevated temperatures. It was revealed that the MWCNT modification using surfactant substantially improves the tensile performance of the epoxy adhesive at temperatures above the T _g of the neat epoxy.     

Properties and processing characteristics of dielectric-filled epoxy resins

A family of casting composites, epoxy resins with mineral fillers, having a range of electrical properties, are being developed. In such composites, the dielectric constant is controlled primarily by varying the filler material in composition and proportions. The present work reports on the mechanical properties of composites made with two types of filler, an alumina powder (XA3500 from ALCOA) and a BaTiO₃/TiO₂ ceramic powder (ATD-50 from Ampex). Dependence of mechanical properties on curing agents was also determined. Filler contents from 0 to 40 percent volume were used. Epoxy systems contained single epoxy resin with both amine and anhydride hardeners. Processing of the anhydride-cured systems was easier than that of the amine-based systems because of their lower viscosity and longer gel time of the former. However, the anhydride-cured systems required higher processing temperatures. Curing kinetics and molecular bonding were investigated using a combination of differential scanning calorimetry, dynamic mechanical thermal analysis, and scanning electron microscopy. Activation energies of 11.2 kcal/mole and 12.1 kcal/mole were obtained for the curing of the amine-based and the anhydride-based composites respectively, and a small difference in the glass transition temperature was also observed. These effects can be attributed to the difference in the structure of the curing agents. The epoxy resin cured with NMA is less ductile compared with those cured with MTHPA or MHHPA due to slight chemical modification on the ring structures. This dependence of ductility on curing agent was observed in specimens with different filler contents. Although the presence of the filler materials was found to enhance the mechanical properties of the epoxy, the fracture mode in these materials is still brittle.     

Boron difluoride chelates as catalysts for curing epoxy resins

BF₂-chelates are powerful polymerisation catalysts for epoxy resins; derivatives of β-diketones and of β-ketoamides are especially interesting as ‘latent’ hardeners. Comparison of these metal halide chelates with BF₃-amine complexes in various epoxy systems has shown the chelates to be superior in several respects. Particular advantages are the ease with which reactivity can be varied by choice of chelate structure and by addition of moderators, and the reduced sensitivity to atmospheric moisture during cure. Use of BF₂-Chelates also gives cured products with better electrical properties, and in many cases better mechanical properties, than with BF₃-amine complexes.