Viscoelastic properties of epoxy resins. III. Effect of molecular weight of antiplasticizers in highly crosslinked antiplasticization system

Antiplasticization, defined as an increase in glassy modulus of polymers upon dilution with low molecular weight compounds, was studied for highly crosslinked epoxy resins. A series of oligomers with average molecular weight 500–4000 was incorporated with epoxy resins for elucidating the effect of their molecular weight on antiplasticization. The antiplasticization was found to be dependent on the molecular weight of the oligomers used. It was most evident when the lowest molecular weight oligomer was incorporated, and seems to be suppressed by increasing molecular weight of the oligomers. These results were discussed in correlation with the behavior of dynamic mechanical properties and the changes in specific volume as a function of the epoxy resin and diluents.     

Thermal properties of epoxy resins crosslinked by an aminated lignin

Aminated lignin possessing significant amount of reactive amino groups was studied as a curing agent of epoxy resin. Fourier transform infrared spectroscopy results proved the reactivity of the aminated lignin with the epoxy resin. Both appearance features and scanning electron microscopy images indicated that the transparent and homogeneous epoxy resin films could be formed with the aminated lignin less than 40% in the hardener mixture. In addition, thermogravimetric analysis and dynamic thermomechanical analysis results revealed that the epoxy resin cured by aminated lignin had better thermal stability compared with ones cured by a common hardener. The mass loss of the epoxy resin cured by the aminated lignin before 300°C was small around only 2.5%. The T_g (the glass transition temperature) of epoxy resin sample after cured by mixed hardener increased from 79°C to 93°C. The obvious difference (70–84°C) of T_d (the thermal deformation temperature) was also observed from the samples with and without the aminated lignin after cured at a high temperature. POLYM. ENG. SCI., 55:924–932, 2015. © 2014 Society of Plastics Engineers     

The glass transition temperatures of highly crosslinked networks: Cured epoxy resins

The glass transition temperatures of a series of cured epoxy resins have been determined, using broad line proton n.m.r. and stress-strain measurements. The networks were prepared by curing bis-phenol-A epichlorohydrin epoxy resins, I, with the stoichiometric amount of diaminodiphenyl methane (DDM), II. The length of the network chains was varied by a factor of 17. The glass transition temperatures ranged from 367 to 410K and were found to be linearly related to the reciprocal of the number average molecular weight, $\text{M}\text{?}{}_{\mathrm{n}}$, of the pre-polymer molecules. A theory is presented in which each component of the network has an associated thermodynamic transition in the pure state. Development of this theory predicts a limiting linear relationship between glass transition temperature and the reciprocal of the number average molecular weight of the network chains between the DDM crosslinks.     

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     

Computer simulation of structure and properties of crosslinked polymers: application to epoxy resins

In this work, a methodology has been developed for construction of atomistic models of crosslinked polymer networks. The methodology has been applied to low molecular weight water soluble epoxy resins crosslinked with different curing agents that are being considered for use as a primer coating on steel. The simulations allowed the crosslink density and the amount of free crosslinking sites in the coatings to be predicted. Shrinkage of the resin upon curing was reproduced by the simulation. In addition, the barrier properties of the model coatings were estimated. The interface between an inorganic substrate and cured epoxy resin has been constructed and the strength and molecular mechanisms of adhesion have been revealed. The developed methodology has a potential to significantly impact on the design and development of new coatings with improved barrier and adhesion properties.     

Chemorheology of crosslinked epoxy resins subjected to large strains

The previously reported work on chemorheological changes occurring in two epoxy resins when aged under large strains at temperatures above their T_g was extended. Ageing times up to 100 h were applied and the phenomenon of orientation occurring in these samples was investigated, mainly by measurements of their birefringence index. It was found that the orientation of the air-aged samples, when studied at room temperature, depended on the ageing conditions (strain, temperature, time) and was not uniform throughout the bulk of the samples, being higher in two narrow external layers and lower in the core layer. After reheating oriented samples in air without strains for different lengths of time at temperatures above T_g, disappearance of the orientation in the external layers was observed. This is probably due to oxidation and degradation occurring in the network and scission of part of the network links, mainly those strained due to orientation. The decrease of orientation, especially in the inner layers of the polymer samples, was much slower when the reheating of the oriented- and unstrained samples took place in an inert atmosphere, this showing again the great influence of oxygen when it participates in ageing and heating processes of epoxy resins.     

Chemorheological changes in crosslinked epoxy resins subjected to large strains

Chemorheological changes in epoxy resins subjected to large strains while heated were observed earlier, but the subject was not treated quantitatively. This paper describes the investigation of an epoxy resin—Epon 826, of known chemical, structure, crosslinked with two different amines. By means of a simple calibrated apparatus, the epoxide samples were subjected to large strains in a non-oxidizing atmosphere and in air. They were heated to different temperatures for various times; the results were compared with data obtained from unstrained samples kept in otherwise the same conditions. The results show a linear-logarithmic relation between the torsion modulus, G₍₁₀₎ and the time of applied strain at a certain temperature and also a linear relation between G₍₁₀₎, and 1/T°K, both above T₍₁₀₎. The results obtained in swelling experiments support the data from 10 sec torsional modulus vs temperature measurements. An increase in the amount of solubles and in M_c is observed on extending the time of heating. A clear difference in properties between the strained and unstrained samples, kept in otherwise the sane conditions, is observed and the contribution of the applied strain to chemorheology has been shown. Both systems of crosslinked Epon 826 showed the same general behavior, although the specific data were different.     

Synthesis and properties of phosphorus‐containing advanced epoxy resins. II

Two phosphorus‐containing diacids were synthesized from 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐oxide (DOPO) and either maleic acid or itaconic acid and then reacted with diglycidyl ether of bisphenol A (DGEBA) to form two series of advanced epoxy resins. Reaction conditions, such as reaction time, temperature and catalyst, are discussed in this article. After curing with 4,4'‐diaminodiphenyl sulfone (DDS), thermal properties of cured epoxy resins were studied using dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA). The flame retardancy of cured epoxy resins was evaluated using a UL‐94 measurement. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 228–235, 2000     

Antiplasticization. II. Characteristics of antiplasticizers

By the incorporation of certain types of additives in bisphenol A polycarbonate, the modulus and tensile strength of the films are increased and the elongation is decreased. This effect is called antiplasticization, because the opposite results are obtained on plasticization—decreased modulus and tensile strength and increased elongation. A study of these additives indicated that antiplasticizers are compounds which are comptiable with the polymer and which (1) contain polar atoms such as halogen, nitrogen, oxygen, or sulfur, (2) contain at least two nonbridged rings, (3) have a glass transition temperature greater than ?50°C., and (4) have one dimension less than about 5.5 A. in at least 65% of the length of the molecules.     

Fracture and mechanical properties of epoxy resins and rubber-modified epoxy resins

Fracture and mechanical property data on a wide range of epoxy resin systems are presented. The extent to which toughening can be induced by heterophase rubber inclusions depends more on the curing agent used than on the resin component. The greatest improvements in toughness were obtained by rubber modification of epoxy resins cured with an anhydride. A preformed ABS polymer can be used to toughen many epoxy resin systems. With one major exception (where a large improvement was found) only small changes in tensile properties occur when small amounts of rubber are present.     

Water in epoxy resins Part II. Diffusion mechanism

A model is proposed for the diffusion mechanism of water in glassy epoxy resins. The polymer network is assumed to consist of two regions in which water molecules possess different mobilities. By considering the distribution of water molecules among these regions it is possible to describe the concentration dependency of the diffusion coefficient in the sorption and resorption processes. The diffusion coefficient becomes constant when the sorption temperature is close to the effective glass transition temperature of the epoxy-water binary mixture. An explanation of this effect is also provided by the model.     

Reversible crosslinking in epoxy resins. II. New approaches

Epoxy resins of varying epoxy equivalent weight were crosslinked with 4,4*-dithiodianiline (DTDA) at a 2/1 mole ratio. Resin of equivalent weight 190 (Epon 828) was cured with DTDA at mole ratios varying from 2/1 to 1.25/1. Properties of cured resins were evaluated and their reduction was investigated. Under the conditions used, reduction to the point of complete solubilization was possible providing crosslink density did not exceed a threshold value tentatively established as M_c = 400 ?; 500. Recurring of reduced resins was accomplished by oxidation and by reaction of thiol groups with polyfunctional reagents, including bismaleimides and polyepoxides.     

Viscoelastic properties of epoxy polymer concrete

The viscoelastic nature of polymer concrete was investigated utilizing an epichlorohydrin/bisphenol A-type epoxy resinaggregate system. Compression specimens were tested for linearity of viscoelastic behavior, the effect of size of mass on creep, and for determination of the specific creep compliance and the associated elastic modulus. The creep compliance was determined by least squares curve fitting of the experimental creep data. Collocation, a numerical Laplace transform inversion routine, was used to develop the equation for the relaxation modulus.     

Polyurethane-crosslinked epoxy resins. II. Compatibility and morphology

The polyurethane(PU)-crosslinked diglycidyl ether of bisphenol A (DGEBA) was prepared from the reaction of the pendent secondary hydroxy group in DGEBA with the isocyanate-terminated group present in the PU prepolymer. Though the PU acted as a crosslinking agent for the DGEBA, the morphology and dynamic mechanical behaviour of these PU-crosslinked DGEBA could be different among themselves depending on the type and molecular weight of polyols employed in the PU. There was no significant shift in the loss modulus (E″) peaks of the DGEBA crosslinked with the polyester-type and polyether-type polyurethanes, but the intensity of the E″ peaks increased as the polyester-type PU employed. The morphology of this polyester-type PU-crosslinked DGEBA exhibited a homogeneous one-phase system, while the DGEBA crosslinked with the polyether-type PU showed a two-phase morphology with the PU particles dispersed in DGEBA matrix.     

Viscoelastic behavior of highly crosslinked poly(acrylic acid)

The effects of crosslinking upon the dynamic mechanical properties and swelling behavior of poly(acrylic acid) (PAA) have been studied. Materials were prepared by the free radical copolymerization of acrylic acid with varying amounts of the tetrafunctional monomer allyl acrylate (ALA). The results indicated a linear dependence of the glass-transition temperature (T_g) on composition, T_g increasing by ～43°C over the mole fraction range X = 0?0.37 ALA. Room temperature (25°C) modulus values, as determined by both dynamic and compression methods, were inversely proportional to the initial concentration of ALA. The degree of network formation has been characterized in terms of the molecular weight between crosslinks M_c, and the influence of this parameter on the swelling ratio was discussed.     

Catalyzed crosslinking of highly functional biobased epoxy resins

Crosslinking reactions involving epoxy homopolymerization of 100% biobased epoxidized sucrose esters (ESEs) were studied and the resulting coatings properties were compared against epoxidized soybean oil (ESO) and petrochemical-based soybean fatty acid ester resins. The low viscosity of ESE resins allowed for formulations to be developed with minimal volatile organic content. ESEs were found to have superior coatings properties, compared to ESO and the petrochemical-based soybean esters, attributable to a higher glass transition temperature (T _g) and a higher modulus. The rigid sucrose core on ESEs provided an increase in coating performance when compared to coatings from epoxidized resins synthesized with tripentaeryithritol as a core. The degree of conversion and optimization of the curing conditions were studied using differential scanning calorimetry (DSC). Thermal analysis of cured coatings was performed using DSC, dynamic mechanical analysis, and thermogravimetric analysis. In order to further enhance the coatings properties, small amounts of bisphenol A epoxy resin were added which resulted in higher moduli and T _gs.     

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.     

Amine/epoxy stoichiometric ratio dependence of crosslinked structure and ductility in amine-cured epoxy thermosetting resins

Epoxy-amine thermosetting resins undergo different reactions depending on the amine/epoxy stoichiometric ratio (r). Although many desirable properties can be achieved by varying the stoichiometric ratio, the effects of the variation on the crosslinked structure and mechanical properties and the contribution of these factors to the ductility of materials have not been fully elucidated. This study investigates the brittle-ductile behavior of epoxies with various stoichiometric ratios and performs curing simulations using molecular dynamics (MD) to evaluate the crosslinked structures. The molecular structure is predominantly branched in low-stoichiometric ratio samples, whereas the chain extension type structure dominates the high-stoichiometric ratio samples. As a result, the higher-stoichiometric ratio samples enhances the ductility of materials and the elongation at break increases form 1.4% (r = 0.6) to 11.4% (r = 1.4). Additionally, the tensile strength (105.4 MPa) and strain energy (7.96 J/cm³) are maximum at r = 0.8 and 1.2, respectively. On the other hand, the Young's modulus is negatively impacted and it decreased from 4.2 to 2.7 GPa with increasing stoichiometric ratio.     

Synthesis and thermal properties of ester‐type crosslinked epoxy resins derived from lignosulfonate and glycerol

Among various biomass‐based components, both lignin and glycerol are important, since they are abundantly produced as by‐products in industrial processes. Accordingly, in the present study, new types of crosslinked epoxy resins were synthesized from lignin and glycerol. Polymers derived from two types of lignin‐based crosslinked epoxy resins were prepared through two‐step reactions, ester‐carboxylic acid derivative preparation followed by crosslinked epoxy resin preparation, in order to establish a crosslinked epoxy resin system in which glycerol units were included. The resins obtained were labeled as follows: series 1, lignosulfonate‐glycerol polyacid (Ser1LSGLYPA); and series 2, glycerol diglycidyl ether (Ser2GLYDGE). The functional groups of the resins were analyzed using Fourier transform infrared spectrometry. The thermal properties of the resins were analyzed using differential scanning calorimetry and thermogravimetry. The glass transition temperature of the crosslinked epoxy resins increased with increasing LSGLYPA and GLYDGE contents for Ser1LSGLYPA and Ser2GLYDGE, respectively. The thermal degradation temperature for Ser1LSGLYPA and Ser2GLYDGE did not show significant change, suggesting that the crosslinked epoxy resins were thermally stable. The mass residue at 500 °C was not affected by the changes of LSGLYPA and GLYDGE contents. Copyright © 2009 Society of Chemical Industry     

Rubber-modified epoxy resins: 1. Equilibrium physical properties

Dilatometry, thermal mechanical analysis, differential scanning calorimetry, electron microscopy and mechanical measurements are reported on a series of rubber-modified epoxy resins. The data obtained is discussed in terms of the interplay of molecular mobility, network and domain structure of the resins. Three transitions were identified; two being assigned respectively to the onset of motion of the acrylonitrile/butadiene units and epoxy units. The higher temperature transition is more difficult to assign, but appears to be associated with specific interactions involving the interface between the epoxy and acrylonitrile/butadiene interfaces or the crosslinked regions of the epoxy resin.