Physical aging of linear and network epoxy resins

Network and linear epoxy resins principally based on the diglycidyl ether of bisphenol-A and its oligomers have been prepared and studied. Both diamine and anhydride crosslinking agents were utilized. In addition, some rubber modified epoxies and a carbon fiber reinforced composite was investigated. All of these materials display time-dependent changes in many of their properties when they are stored (following quenching) at temperatures below their glass transition temperature (sub-T_g annealing). For example, the degree of stress relaxation for a given time period is observed to decrease in a linear fashion with the logarithm of time during sub-T_g annealing. Young's modulus and yield stress were also found to increase ire physical aging. Solvent sorption experiments initiated after different sub-T_g annealing times have demonstrated that the rate of solvent uptake can be indirectly related to the free volume of the epoxy resins. The effect of water on the physical aging of these epoxy resins was not found to be a significant variable. Residual thermal stresses were also found to have little effect on the physical aging process, although this variable was not studied in detail. Finally, the physical aging process also affected the sub-T_g properties of uniaxial carbon fiber reinforced epoxy material and the effects were as expected. The importance of the recovery or physical aging phenomenon, which affects the durability of epoxy glasses, is considered in view of the widespread applications for these resins as structural materials.     

A dynamic mechanical study of the curing reaction of two epoxy resins

The curing behavior of two commercially formulated epoxy resins composed of the tetrafunctional amine dicyandiamide and with differing epoxy components, 4,4′-bisglycidylphenyl-2,2′-propane and the tetraglycidyl ether of methylene dianiline, is characterized by dynamic spring analysis. This supported viscoelastic technique is well suited to the determination of the onset of gelation under isothermal conditions but the method is not useful for monitoring later stages of reaction when the resins become more rigid. The activation energy for the curing of the two resins is about 87 kJ/mole (20.7 kcal/mole). Rate constants for the first order curing reaction are given. Additional studies of films cured below the ultimate T_g show that two relaxations can be observed upon heating. The first relaxation occurs near the original isothermal cure temperature with a low activation energy, about 250 kJ/mole, whereas the second relaxation occurs near the ultimate T_g, under the conditions used here, with an activation energy of 500–650 kJ/mole. It is believed that these activation energies provide a unique method of characterizing the molecular mobility of epoxy resins at various states of cure.     

Structure and viscoelastic properties of epoxy resins prepared from four-nuclei novolacs

The relation between the structure and the viscoelastic properties of seven kinds of epoxy resins was studied. Seven tetraglycidylethers were synthesized from four-nuclei novolacs in which the positions of methylene linkage or number of kind of substituents were different. These epoxy compounds were cured with diaminodiphenylmethane as a hardener. From the viscoelastic properties of the fully cured resins with the hardener, characteristic properties such as glass transition temperature (T_g), average molecular weight between crosslinking points (M̄_c), and front factor (ϕ) were obtained. It was concluded that higher linearity in the main chain of epoxy resins gave a cured resin with a higher T_g, a smaller M̄_c, and a larger ϕ.     

Rheological characterization of 4,4′-bis(diethylamino)benzophenone-functionalized polybutadienes with low and medium vinyl content

An experimental study was undertaken to investigate the thermomechanical properties of a certain epoxy/amine configuration. The basic structure of all the epoxies was the same—DGEBA—and the curing agent used was PACM 20. By varying the epoxy prepolymer molecular weight and the stoichiometry between epoxy and amine, a range of different epoxy networks were produced. Glass transition temperatures were evaluated by using differentil scanning calorimetry (DSC). Modulus values as well as an alternative T_g determination were provided by dynamic mechanical analysis (DMA). Coefficients of thermal expansion were obtained from thermomechanical analysis (TMA). The tensile tests conducted at room and elevated temperatures provided additional modulus data along with the yield point, tensile strength, and elongation at break data. Property vs. stoichiometry curves exhibited a maximum for the glass transition temperature and the over the T_g modulus at the stoichiometric point. On the other hand, the under T_g modulus showed a minimum at the stoichiometric point. The results of the yield strength show remarkable similarity with the results of the modulus. Strength and elongation at break do not show clear trends, but a much different behavior is exhibited between room and elevated temperatures. © 1996 John Wiley & Sons, Inc.     

Bisphenol-A bimodal epoxy resins. Part I: The dynamic mechanical characterization of a 6300 (340/22,500) weight average molecular weight system

By using an advanced epoxy resin of 22,500 weight average molecular weight two bimodal systems of 6300 weight average molecular weights were prepared. By altering the curing procedure normally used to cure epoxy resins and high molecular weight resins we have succeeded in minimizing the difficulty associated with preparing bimodal epoxy resin systems. The ultimate T_g of these bimodal systems is associated with the phase morphology and controlled by the curing conditions employed. For the completely phase separated bimodal system a T_g of 473 K is reported and for the partially phase separated system a T_g of 466 K is reported. Equations were developed for predicting the equilibrium shear modulus of these bimodal systems. Theoretical predictions based on these equations were found to be consistent with experimental results.     

New epoxy resins. II. The preparation,characterization, and curing of epoxy resins and their copolymers

A polymer having high aromaticity and/or cyclic ring structures in the chain backbone usually gives high heat resistance and flame resistance. Five glycidyl ether-type epoxy resins are prepared from bisphenol A (DGEBA), 9,9-bis(4-hydroxyphenyl)fluorene (DGEBF), 3,6-dihydroxyspiro-[fluorene-9,9′-xanthane] (DGEFX), 10,10-bis(4-hydroxyphenyl) anthrone (DGEA), and 9,9,10,10-tetrakis(4-hydroxyphenyl)anthracene (TGETA) in order to study structure–thermal stability–flame resistance property relationships. In this study, trimethoxyboroxine (TMB) and diaminodiphenylsulfone (DDS) are employed as the curing agents. The char yield at 700°C under a nitrogen atmosphere and the glass transition temperature (T_g) for the uncured resins decrease according to the sequence TGETA > DGEFX > DGEA > DGEBF > DGEBA. The Tg values for these cured epoxy resins are DGEBA < DGEBF < DGEFX < DGEA. A T_g for the TGETA is not obtainable but would be expected to be the highest. The char yields at 700°C of these cured epoxy resins have the same trend as the uncured resins. DGEBF, DGEFX, DGEA, and TGETA added to the DGEBA system show increases in the char yield, T_g, and oxygen index with increasing concentration of these novel epoxy resins.     

Mechanical and thermal properties of epoxy resins with reversible crosslinks

The tensile properties: Young's modulus, ultimate tensile strength, ultimate elongation, the glass transition temperature, and the dynamic mechanical properties (dynamic shear modulus (G'), loss tangent (Tan δ)), of three epoxy resins (Epon 828, Epon 836, Epon HPT 1071) cured with the disulfide-containing crosslinking agent—4.4-dithiodianilme (DTDA) have been characterized. The results show that DTDA is a satisfactory crosslinking agent for the epoxide resins that have been studied as compared to the well-known curing agent methylene dianiline (MDA). There are no significant differences between the properties of Epon 828 cured with DTDA at stoichiometric ratio (2:1) and Epon 828 cured with DTDA at small amine excess ratio (1.75:1). The glass transition temperature of the cured tetrafunctional epoxy resin Epon HPT 1971 (235°C) is significantly higher than that of difunctional epoxy resins such as Epon 828 (T_g–175°C), but the product is too brittle to be used without plasticizer.     

Epoxidized soybean oil modified using fatty acids as tougheners for thermosetting epoxy resins: Part 2—Effect of curing agent and epoxy molecular weight

A series of bio-rubber (BR) tougheners for thermosetting epoxy resins was prepared by grafting renewable fatty acids with different chain lengths onto epoxidized soybean oil at varying molar ratios. BR-toughened samples were prepared by blending BRs with diglycidyl ether of bisphenol A resins, Epon 828 and Epon 1001F, at different weight fractions and stoichiometrically cured using an amine curing agent, 4, 4′-methylene biscyclohexanamine (PACM). Fracture toughness properties of the unmodified and BR toughened polymer samples—including critical strain energy release rate (G_Ic), and critical stress intensity factor (K_Ic)—were measured to investigate the toughening effect of prepared BRs. It was found that the degree of phase separation and toughening were more controllable relative to similar polymers cured using the aromatic curing agent Epikure W, and the use of higher molecular epoxy resins produces a synergistic effect increasing the toughness much more than similar polymers made with lower molecular weight epoxy resins. Average BR domain sizes ranging from 200 to 900 nm were observed, and formulations with G_Ic, values K_Ic as high as 1.0 kJ/m² and 1.4 MPa m^1/2 were attained respectively for epoxy systems with T_g greater than 130°C.     

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.     

Phase separation of poly(ether sulfone imide) modified epoxy resin

Poly(ether sulfone imide)s (PEI) with molecular weight M_n ∼ 10⁴ were synthesized from 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and amine terminated poly(ether sulfone) having molecular weights ranging from M_n ∼ 400 to M_n ∼ 4000. Thus, the PEIs had the same molecular weight but various imide and ether sulfone contents. The PEIs were mixed with a stoichiometric mixture of diglycidyl ether bis-phenol-A (DGEBA)/diamino diphenyl sulfone (DDS). The effect of PEI on the curing reaction of DGEBA/DDS and the morphology of the polymer blend were studied by differential scanning calorimetry (DSC) and optical microscopy. In the DGEBA/DDS/PEI blend with a fixed PEI molecular weight and PEI concentration but with various imide content, the experimental data revealed the PEI with a higher content of ether sulfone had a lower T_g and a better compatibility with solvents and epoxy resins; the curing reaction rate of DGEBA/DDS/PEI was faster for PEI with a higher imide content; the DSC data of cured DGEBA/DDS/PEI showed two T_gs, indicating phase separation between PEI and cured epoxy resins; and the data of optical microscopy showed that the compatibility of PEI with epoxy resins increased with the content of ether sulfone in PEI. © 1996 John Wiley & Sons, Inc.     

Properties of a reactive‐graphitic‐carbon‐nanofibers‐reinforced epoxy

Our previous studies showed that herringbone graphitic GNFs surface‐derivatized with reactive linker molecules bearing pendant primary amino functional groups capable of binding covalently to epoxy resins. Of special importance, herringbone GNFs derivatized with 3,4′‐oxydianiline (GNF‐ODA) were found to react with neat butyl glycidyl ether to form mono‐, di‐, tri‐, and tetra‐glycidyl oligomers covalently coupled to the ODA pendant amino group. The resulting reactive GNF‐ODA (butyl glycidyl)_n nanofibers, r‐GNF‐ODA, are especially well suited for reactive, covalent incorporation into epoxy resins during thermal curing. Based on these studies, nanocomposites reinforced by the r‐GNF‐ODA nanofibers at nanofiber loadings of 0.15–1.3 wt% were prepared. Flexural property of cured r‐GNF‐ODA/epoxy nanocomposites were measured through three‐point‐bending tests. Thermal properties, including glass transition temperature (T_g) and coefficient of thermal expansion (CTE) for the nanocomposites, were investigated using thermal mechanical analysis. The nanocomposites containing 0.3 wt% of the nanofibers gives the highest mechanical properties. At this 0.3‐wt% fiber loading, the flexural strength, modulus and breaking strain of the particular nanocomposite are increased by about 26, 20, and 30%, respectively, compared to that of pure epoxy matrix. Moreover, the T_g value is the highest for this nanocomposite, 14°C higher than that of pure epoxy. The almost constant change in CTEs before and after T_g, and very close to the change of pure epoxy, is in agreement with our previous study results on a chemical bond existing between the r‐GNF‐ODA nanofibers and epoxy resin in the resulting nanocomposites. POLYM. COMPOS., 28:605–611, 2007. © 2007 Society of Plastics Engineers     

Synthesis and properties of urethane elastomer-modified epoxy resin having hydroxymethyl group

Synthesis and properties of urethane elastomer-modified epoxy resins were studied. The urethane elastomer-modified epoxy resins were synthesized by the reaction of a 4-cresol type epoxy compound having hydroxymethyl groups (EPCDA) with isocyanate prepolymer. The structure was identified by IR, ¹H NMR and GPC. These epoxy resins (EPCDATDI) were mixed with a commercial epoxy resin (DGEBA) in various ratios. The mixed epoxy resins were cured with a mixture of 4,4′-diaminodiphenylmethane and 3-phenylenediamine (molar ratio 6:4) as a hardener. The curing behaviour of these epoxy resins was studied by DSC. The higher the concentration of EPCDATDI, the higher the onset temperature and the smaller the rate constant (k) of the exothermic cure reaction were. It was considered that the ratio of hydroxymethyl group to epoxide group was very small and the molecular weight of EPCDATDI was large. Therefore, the accelerating effect of the hydroxymethyl group on the epoxide–amine reaction was cancelled by the retardant effect of increased molecular weight and viscosity, and decreased molecular motion. Toughness was estimated by Izod impact strength and fracture toughness (K_1C). On addition of 10 wt% EPCDATDI with low molecular weight (M?_n 6710, estimated by GPC using polystyrene standard samples), Izod impact strength and K_1C increased by 70% and 60%, respectively, compared with unmodified epoxy resin. Glass transition temperatures (T_g) for the cured epoxy resins mixed with EPCDATDI measured by dynamic mechanical spectrometry were the same as those of unmodified epoxy resin. The storage modulus (E′) at room temperature decreased with increasing concentration of EPCDATDI. Toughness and dynamic mechnical behaviour of cured epoxy resin systems were studied based on the morphology.     

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.     

Effect of time-temperature path of cure on the water absorption of high Tg epoxy resins

The effect of different time—temperature paths of cure on the water absorption of high T_g epoxy resins has been investigated. The resins were cured isothermally for different times, with the following results: as extent of cure increased, the glass transition temperature (T_g) increased, the room temperature (RT) modulus decreased, the RT density decreased, the RT diffusion coefficient appeared to decrease, and the RT water absorption increased. The decrease in RT density is related to an increase in free volume, which controls the amount of water absorbed. A qualitative model accounts for the increase in RT free volume with increasing cure. The model is based on a restricted decrease of free volume on cure due to the rigid molecular segments in the cured resin systems. The sorption isotherms can be characterized by the dual mode theory at low activities but at high activities the sorption is complicated by penetrant clustering. A thermodynamic approach, independent of the absorption model, can correlate sorption data at different temperatures. The diglycidyl resin was also cured for extended times at three temperatures, in an effort to achieve full cure at each temperature. For these, the higher the cure temperature, the lower the RT density, which could result from the lower initial density of materials cured at higher temperatures. The equilibrium water absorption increased with increasing cure temperature, consistent with the decrease in RT density. The systems studied were a diglycidyl ether of bisphenol A cured with an aromatic tetrafunctional diamine, trimethylene glycol di-p-aminobenzoate (T_g∞ = 156°C), and a triglycidyl ether of tris(hydroxyphenyl)methane cured with the same amine (T_g∞ = 268°C).     

Simultaneously increasing cryogenic strength, ductility and impact resistance of epoxy resins modified by n-butyl glycidyl ether

Epoxy resins are increasingly used in cryogenic engineering areas due to rapid developments of spacecraft and superconducting cable technologies as well as large cryogenic engineering projects (e.g., International Thermonuclear Experimental Reactor). Cryogenic mechanical properties are important parameters for epoxy resins to be employed in such areas. In this paper, a traditional reactive aliphatic diluent, namely n-butyl glycidyl ether (BGE, JX-013) with a low viscosity, was used to modify diethyl toluene diamine (DETD)-cured diglycidyl ether of bisphenol-F (DGEBF) epoxy system for enhancing cryogenic mechanical properties at liquid nitrogen temperature (77 K). The results showed that the cryogenic strength, ductility and impact resistance (impact strength) have been simultaneously enhanced by the addition of BGE with appropriate contents. Moreover, the comparison of the mechanical properties between 77 K and room temperature (RT) indicated that at the same composition, the tensile strength and Young's modulus at 77 K were higher than those at RT but the failure strain and impact resistance showed the opposite results. Finally, differential scanning calorimetry (DSC) exhibited that the glass transition temperatures (T_g) of the epoxy resins decreased with increasing the BGE content.     

Synthesis and thermally stimulated current study of molecular relaxations of the grafted polymethylphenylsiloxane segment in thiodiphenol-modified epoxy resin

A series of grafted polymethylphenylsiloxane (PMPS) segments in thiodiphenyl-containing epoxy resin (ESTP) was prepared. The structure was evaluated by IR, ¹H-NMR, and ¹³C-NMR measurements. DSC measurements on the grafted ESTP epoxy resins showed a decreasing trend for T_g with increasing content of PMPS siloxane. The TSC measurements further confirmed this trend. This result suggests that the matrix of ESTP copolymer was less rigid with grafting of PMPS oligomer, due to the Si—O—C linkage at the opened glycidyl ether chain located approximately between the BPA and 4,4′-thiodiphenoxyl chains in the epoxy backbone. A new and broad sub-T_g transition appeared at −60°C in the TSC spectrum was observed for the cooperative motion of this siloxane moiety. All copolymers showed sub-T_g relaxations of γ- and δ-modes, observed at −100 and −130°C, respectively. These two relaxations may be attributed to the motions of BPA and the terminal groups in the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1523–1530, 1998     

Thermomechanical properties of arylamine‐based benzoxazine resins alloyed with epoxy resin

Effects of epoxy resin on various arylamine‐based benzoxazine resins, i.e., aniline (BA‐a), m‐toluidine (BA‐mt), and 3,5‐xylidine (BA‐35x), have been investigated. Processing windows of BA‐35x, BA‐mt, and BA‐a were found to be widened with the amount of the epoxy. Gel points of benzoxazine‐epoxy resin mixtures can be predicted by an Arrhenius equation, e.g., gel time of BA‐35x and epoxy mixture at 70:30 mass ratio can be estimated by t_gel = 0.7012 × 10^?7 exp (10.563/T). Glass transition temperature (T_g) of BA‐a and BA‐mt alloyed with epoxy exhibited a synergistic behavior with the maximum T_g value at the benzoxazine‐epoxy composition of 80:20 mass ratio. However, in the BA‐35x and epoxy mixture, the decreasing trend in T_g from 241°C to 223°C with an addition of epoxy was observed. Furthermore, flexural strength and strain‐at‐break of those alloys were found to increase with increasing amount of the epoxy while modulus increased with the polybenzoxazine mass fraction. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Electric strength of epoxide resins and its relation to the structure

Epoxide resins having various ratios of ether and ester bonds were investigated as to the relation between electric strength and polymer characteristics. The electric strength over a wide range of temperature is presented here. A marked reduction of strength characteristics of the epoxide resins occurs at a critical temperature indistinguishable from the glass transition temperature T_g, which is related to the free volume and molecular relaxation process. At temperatures exceeding T_g, the electric strength has a strong dependence on polymer structure, film thickness, and applied pulse width. This behavior is considered to obey the thermal breakdown mechanism, and it is assumed that the ion is important in the precursory region of electric breakdown.     

Organophosphorous additive for fortification,processibility, and flame retardance of epoxy resins

Amine‐cured epoxy resins are prepared containing an aliphatic phosphonate additive which aids in the processing of the epoxy by reducing the viscosity of the resin mixture. The additive also acts as an antiplasticizer in the cured epoxy by increasing the modulus and yield strength under uniaxial tension and compression. Additionally, phosphonates are known to behave as flame retardants and dimethyl methyl phosphonate (DMMP) demonstrates a reduction in the rate of thermal degradation and the heat‐release rate upon pyrolysis. Addition of the antiplasticizer reduces the T_g and suppressed β‐relaxations, while effectively increasing the density of the material. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 302–309, 2002; DOI 10.1002/app.10329     

New epoxy resins. I. The stability of epoxy–trialkoxyboroxines triaryloxyboroxine system

A polymer with high aromaticity and/or cyclic ring structures chain backbone usually has high heat, thermal, and flame resistance. Two diglycidyl ethers of bisphenols were prepared from 4,4′ isopropylidenediphenol (DGEBA) and 9,9-bis(4-hydroxyphenyl) fluorene (DGEBF) for evaluation. Four boroxines—trimethoxyboroxine (TMB), triethoxyboroxine (TEB), triisopropoxyboroxine (TIPB) and triphenoxyboroxine (TPB)—were used as the curing agents. DGEBA and DGEBF cured with various boroxines indicate that the trend for their respective glass transition temperature (T_g's), degradation temperatures (T_d's), and gel fractions are TMB-cured epoxy ≈ TEB-cured epoxy < TIPB cured epoxy < TPB cured epoxy. The DGEBF system usually has a higher T_g, T_d, gel fraction, oxygen index (OI), and char yield than the related DGEBA system. DGEBF/DGEBA (80/20 mol ratio) shows a synergistic effect in regard to char formation. This effect exists not only in the copolymer system but also in blended homopolymers of the separately cured resins. A modified mechanism for the polymerization of phenyl glycidyl ether (PGE) with TMB has been proposed.