Studies in the molecular weight distribution of epoxide resins. II. Chain branching in epoxide resins

The extent of side chain branching in epoxide resins based on bisphenol A has been determined by nuclear magnetic resonance spectrometry. The results indicate that for the resins studied in this paper, the extent of branching is very small. The number of branch points varies between 0.09 to 0.06 per molecule for epoxide resins whose number-average molecular weight lies between 1500 and 4000.     

Studies in the molecular weight distribution of epoxide resins. I. Gel permeation chromatography of epoxide resins

The molecular weight distributions as measured by gel permeation chromatography of solid epoxide resins made by the direct addition of epichlorohydrin to bisphenol A (the “taffy” process) and by the reaction of low molecular weight liquid epoxide resins whose main constituent is the diglycidyl ether of bisphenol A with bisphenol A (the “advancement” process) have been compared with the theoretical distribution calculated by the application of Flory statistics. The model used for predicting the molecular weight distribution has been shown to be too simple to describe the real size distribution of these resins. For resins prepared by the “taffy” process, incompleteness of reaction, the presence of monofunctional epoxides, and the possibility of branching reactions through the epoxide–hydroxyl reaction lead to a distribution that more nearly resembles one calculated for a resin having a higher epoxide value than that actually measured. In the case of resins prepared by the “advancement” process, the presence of small amounts of the higher oligomeric diepoxides and monofunctional epoxides in the starting material used for the synthesis lead to complex molecular weight distributions that are not easy to deduce theoretically. The experimentally determined molecular weight distributions for the higher molecular weight epoxide resins (epoxide value <2 eq/kg) made by the “advancement” process resemble more nearly those calculated for resins having lower epoxide values than those actually measured.     

Very slow crack growth during osmosis in epoxy and in polyester resins

It has been demonstrated that osmotic pressure filled cracks in both epoxy and polyester resins are elastic cracks. Use of classical formulas for elastic cracks has enabled estimates to be made of the time dependence of Young's modulus for both resins. Use of linear elastic fracture mechanics formulas has enabled stress intensity factors to be determined from measurements of crack profiles. Radical crack growth rates are small, in the range 10^?12–10^?9 ms^?1 for hot water tests, and remain constant over a wider range of stress intensity factor, from 0.3 to 0.8 MPa.m^1/2. To a first approximation, constant radial growth rate is compatible with a diffusion controlled mechanism. However, analysis of the data indicates an activation energy of ～50 kcal. Some evidence is presented for concluding that, in polyesters at least, the true nature of crack propagation can be by way of slip/stick.     

Small-angle neutron scattering from branched epoxide resins

Small-angle neutron scattering experiments in the range of q² from 0.01 to 25 nm⁻² have been carried out on branched epoxide resins based on bisphenol-A at the Institute Laue—Langevin (I.L.L) in Grenoble (q=(4π/λ) sin(θ/2)). Measurements were made with six samples in the range of M_W from 1500 to 19 000 and four concentrations between 1.3 and 10% (w/w) in deuterated diglyme. The results are as follows: (i) The mean square radius of gyration follows a relationship S²_z=4.69×10⁻⁴M^1.20_W (nm²). (ii) In all cases fairly large second virial coefficients A₂ are obtained which, however, decrease strongly with molecular weight. Above M_W=2500, the virial coefficient follows the relationship A₂=1.6M^−0.85_W (mol cm³g⁻²). (ii) The reciprocal particle scattering factor as a function of q² exhibits only a slight upturn and otherwise shows the behaviour of a randomly branched polycondensate. The slight upturn is discussed as being caused by the finite volume of the monomeric unit. Possible reasons for the high exponent in the S²_z versus M_W dependence are briefly discussed.     

Studies in the molecular weight distribution of epoxide resins. III. Gel permeation chromatography of epoxide resins subject to postglycidylation

Unambiguous evidence has been provided for the existence of a branched structure in liquid epoxide resins which have been subjected to postepoxidation with excess epichlorohydrin in the presence of a powerful nucleophilic catalyst, tetramethylammonium chloride. Gel chromatography of the resin on Sephadex LH-20 followed by isolation and identification of the relevant fraction by spectroscopic techniques revealed the presence of a novel trifunctional epoxide having a molecular weight of 680.     

The effect of microstructure on the slow crack growth resistance in polyethylene resins

The slow crack growth (SCG) in high density polyethylene (HDPE) is a phenomenon dominated by crazing. In this work, the crazing was analyzed from a microstructural point of view. PENT (Pennsylvania Edge Notched Tensile) test was chosen to study the evolution of the craze with time for different resins from PE‐80 up to PE‐100 grades. Two different geometries, the standard and an alternative named CDNT (Circumferentially Deep Notched Tensile), were employed. Failure times were correlated with intercrystalline parameters like tie molecules and the molecular weight between entanglements. Experimental results showed good correlations using both direct SCG test (standard PENT and CDNT geometries). Finally, the strain hardening modulus was correlated with PENT failure times. The results disclosed an outstanding correlation for several polyethylene grades from blow molding up to PE‐80, PE‐100, and higher resistant to crack grades. These results permitted an easy‐classifying and ranking method as much to the old polyethylene grades as to the new generation of HDPE resins with a very high SCG resistance. POLYM. ENG. SCI., 55:1018–1023, 2015. © 2014 Society of Plastics Engineers     

Stability of crack propagation in epoxy resins

The propagation of cracks in epoxy resins has been studied using a linear elastic fracture mechanics approach and a double torsion testing geometry. Under constant crosshead displacement rate conditions cracks are found to propagate in an unstable ‘stick-slip’ manner at high temperatures and with low rates of testing whereas at lower temperatures and using higher rates of loading propagation is more stable and cracks propagate in a continuous manner. The presence of liquid water tends to cause a transition from stable to unstable propagation at room temperature. The influence of specimen geometry upon crack stability is also discussed.     

Further aspects of the thermal degradation of epoxide resins

This paper describes an investigation into the degradation of a purified epoxide based on the diglycidyl ether of bisphenol A hardened with p,p′-diaminodiphenylmethane. The method used was that of hot-wire pyrolysis followed by gas chromatography. Special attention was given to the problem of solid residues formed on the pyrolyzer tube, and evidence was found that these probably contain oligomers. Resonance-stabilized free radicals also appear to be formed, and evidence is found to support the idea of dehydration during degradation, originally put forward by Lee. An attempt, based on first principles, is made to explain the degradation of epoxides, using evidence from previous work as well as that described in this paper.     

Studies in the molecular weight distribution of epoxide resins. IV. Molecular weight distributions of epoxide resins made from bisphenol a and epichlorohydrin

The molecular weight distribution of epoxide resins made from bisphenol A and epichlorohydrin at high ratios of epichlorohydrin to bisphenol A are compared with the theoretically predicted distributions for two theoretical models: the “taffy” process A, the direct reaction of epichlorohydrin with bisphenol A; and the “taffy” process B, the self-polymerization of a monoglycidyl ether of bisphenol A followed by postglycidylation. At high ratios of epichlorohydrin to bisphenol A, process B is shown to give more low molecular weight products than process A. Deviations of the experimentally measured distributions from the theoretically predicted distributions for high epichlorohydrin/bisphenol ratios are attributed to the higher reactivity of epichlorohydrin to the phenolic compared with the aryl glycidyl ether functional group. Preliminary kinetic data are presented using a modified gel chromatographic method which enables the separation of most of the intermediates formed in this reaction.     

Shrinkage and internal stress during curing of epoxide resins

The shrinkage and internal stress of bisphenol-type epoxide resins cured with aliphatic α,ω-diamines, H₂N? (CH₂)_m′? NH₂(m′ = 2, 4, 6, and 12), were investigated by measuring the change of density and the strain of the steel ring embedded in the cured resins. Internal stress was found to be induced by the shrinkage occurring in the cooling process from the glass transition temperature (T_g) to room temperature. Shrinkage and internal stress increased with increase in the concentration of network chains and T_g of the cured systems, and then with a decrease in m′ of the curing agents. It appears that the reductions in the concentration of network chains and T_g were necessary to reduce the shrinkage and internal stress caused by the curing.     

Mechanical relaxation mechanism of epoxide resins cured with diamines

The mechanism of mechanical relaxation, which is observed between 50° and 90°C in epoxide resins cured with aromatic and alicyclic diamines, has been investigated by comparing dynamic mechanical properties and chemical structures of these networks. This relaxation is denoted here as the α′ relaxation. The occurrence of the α′ relaxation depends on the existence of p-phenylene groups, and is independent of the degree of cure in the cured epoxide resins. Moreover, the intensity of the α′ relaxation increases linearly with increasing the concentration of p-phenylene groups in the networks. From these results, it is concluded that the α′ relaxation of the cured epoxide resins is attributed to the motion of p-phenylene groups in the network structures.     

Mechanical properties of acid-cured epoxide resins with different network structures

Static and dynamic mechanical properties of cured epoxide resins based on ester bonds, ether bonds, or a mixture of ester and ether bonds were investigated. Their network structures were estimated from the results of gel content before and after saponification, and conversion of functional groups. It was found that cured epoxide resins based on a mixture of ester and ether bonds indicate intermediate properties between the resins based on ester bonds and the resins based on ether bonds. Both dynamic and static mechanical properties were strongly affected by their network density and their segmental structures suggested in this paper.     

The structure of epoxide resins cured by BF3 catalysts

The chemical structure of three-dimensional networks of diglycidylester of hexahydrophthalic acid cured by BF₃-amine adducts was determined by estimation of the degree of polymerization of the polyglycidol isolated after saponification of this crosslinked polymer. Using the kinetic theory of rubber elasticity the modulus of the crosslinked polymer in the rubber region was calculated, which showed a good agreement with the experimentally obtained value.     

Slow crack propagation in a heavily-filled particle reinforced epoxide resin

The fracture properties of two proprietary composite dental restorative materials and a model composite system were studied to determine the effects of filler concentration, exposure to water, and particle/polymer adhesion on subcritical crack propagation. Particle content ranged from 36 to 60 volume percent. The double torsion (DT) test was used to measure relationships between the stress intensity factor (K₁) and the speed of decelerating cracks or the rate of loading in dry and wet materials in air at laboratory conditions. Materials with weak particle/polymer interfaces fractured by continuous crack growth in both dry and wet conditions. In dry and wet materials with strong interfaces, continuous cracking also occurred at the low end of the range of speeds observed (10⁻⁷ to 10⁻³ m/s), but under test conditions of high crack speeds unstable (stick-slip) crack propagation was found in dry specimens and in wet model composites with 41 percent vol, filler. Water had a corrosive effect lowering K_1c for continuous crack propagation. The exponential dependence of K_1c on crack velocity, representing the viscoelastic response of the materials, was positively correlated to the filler concentration and the plasticizing effect of water. Observations on fracture surfaces indicate that low velocity cracks (<10⁻⁵ m/s) propagate through regions of high stress concentrations (interfaces, corners, pores) while at higher crack velocities failure occurs by a combination of interparticle and transparticle fracture.     

The mechanism for occurrence of internal stress during curing epoxide resins

The mechanism for the occurrence of internal stress in the curing cycle of four-functional epoxide resins was investigated in detail. The internal stress in this system was generated at the vitrification point in the course of curing, because the modulus of samples was rapidly increased at this point. After the vitrification point, the internal stress was increased with an increase of the shrinkage in the curing and cooling processes. Moreover, the magnitude of the internal stress in the four-functional resin systems depended on the chemical structure of aromatic diamines used as curing agents. This was explained by the difference of curing shrinkage after vitrification in each system.     

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.     

Curing of epoxide resins by anhydrides of dicarboxylic acids: model reactions

Summary The kinetics of the polyaddition reaction of epoxy resins were studied using the model system phenyl glycidyl ether, phthalic anhydride, tertiary amine. N,N-dimethyl-benzylamine and N,N-dimethylaniline were used as tertiary amines. Furthermore the influence of phenol and benzoic acid was investigated. The oligomer products were separated by high pressure liquid chromatography. A reaction scheme of the copolyaddition and the structure of the products were proposed.     

Effect of tertiary amines on yellowing of UV‐curable epoxide resins

The work reported demonstrates that the yellowness of UV‐curable epoxide resins can be improved by adding certain tertiary amines in appropriately determined amounts. According to the results of our experiments, 2.0 wt% benzoyl peroxide added to a resin effectively enhances the crosslinking density, and phenolic free radicals are produced during UV curing, which consequently induce yellowness via the reaction of oxygen and the free radicals. Imidazole (1‐amine) and tertiary amines, including 1,2‐dimethylimidazole (2‐amine), 2,4,6‐tris(dimethylaminomethyl)phenol (3‐amine), 1‐methylimidazole (4‐amine) and 2‐methylimidazole (5‐amine), were chosen to be added to resins, and their effects on UV conversion and yellowness were investigated. According to the experimental results, tertiary amines in the resin can provide a certain degree of improvement in yellowness index (ΔYI) and color parameter (ΔE*_ab) of the resin sample. Whatever the type of tertiary amine, it is found that the optimum content of amine in resin is 1.0 wt%. Also, among the studied amines, the 3‐amine exhibits the highest UV reactivity and the best efficiency for yellowness improvement with values of Δa*, Δb*, ΔYI and ΔE*_ab as low as ? 1.4, 6.23, 11.27 and 6.48, respectively. Copyright © 2007 Society of Chemical Industry     

The mechanism of the thermal degradation of aromatic amine-cured glycidyl ether-type epoxide resins

A mechanism is proposed for the thermal degradation of aromatic amine-cured glycidyl ether-type epoxide resins which is based on the results of previous work in this field. The significance of the degradation mechanism to the thermal stability of aliphatic amine-cured epoxides and aromatic amine-cured cycloaliphatic epoxides is discussed.     

Chemistry of epoxide resins. XVII. Influence of structure and curing conditions on the density,degree of cure,and glass transition temperature during the curing of epoxide resins

During an investigation of various epoxide resin systems, some cases were found in which the room temperature density decreased with increasing curing temperature and increasing degree of cure. In other systems the density was found to be independent of the curing temperature. In these cases it is possible by deliberately stopping the reaction to measure density values which also decrease as the curing progresses. This unexpected behavior can be explained in a purely physical manner from the pattern of the density changes during an entire curing cycle. The density decrease of the noncured mixture, which is due to the increased curing temperature, outweighs all contraction effects consisting of isothermal chemical and cooling shrinkage, whereby the latter is dependent to a great extent on the glass temperature. In those cases where the glass temperature exceeds the curing temperature, the chemical reactions come to a standstill, when the temperature difference reaches a certain value, i.e. it “freezes chemically”. By means of values that can be measured readily at low temperatures, it is possible to construct diagrams from which the variation of the density at higher temperatures of the curing cycle can be estimated.