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.     

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.     

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.     

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.     

Mechanics of crack growth in epoxide resins

Experiments have been conducted employing tapereddouble-cantilever-beam joints with different epoxide adhesives. Depending on the adhesive employed, crack propagation occurred either (a) in a continuous stable manner with crack propagation velocities in the range 10^?4 to 5 m/s and values of the adhesive fracture energy, G_Ic, being almost independent of the crack velocity, or (b) intermittently in an unstable manner when the initial crack velocity was never less than about 20 m/s and, in some instances, rose to about 450 m/s; values of G_Ic (initiation) increased rapidly with increasing velocity. It is proposed that the amount of localized plastic deformation arising from shear yielding that occurs at the crack tip prior to crack propagation is controlling. Secondly, the longterm strength of stressed, structural adhesive joints has been investigated. The fracture of these joints over eight decades of time is uniquely described by a critical plastic zone size developed at the crack tip at failure.     

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.     

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.     

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.     

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 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.     

A natural frequency approach to study the blown film helicoidal stability of HDPE bimodal resins and its relation to their rheological behavior

A quantitative study of the helicoidal bubble stability (HBS) of five different HDPE bimodal resins processed under the same conditions was conducted to determine their natural frequencies and how these are related to molecular weight distribution features and typical viscoelastic data. The natural frequency , of each resin was obtained by representing the HBS phenomenon as an undamped system under the influence of a harmonic force, whose solution required force balance and Fourier series analyses of video‐recorded oscillations of the bubbles. It was found that was directly related to the stability grading on these bimodal resins established by experimental technicians running the blown film line. The same trends were observed when comparing with the entanglement index, Mw_B/Me, the characteristic retardation time t_67% obtained from recovery compliance data, and the cross‐over point frequency (COP). These trends were not linear, indicating perhaps the existence of a percolation threshold for with respect to the entanglement index and to the mentioned rheological parameters. POLYM. ENG. SCI., 55:2910–2921, 2015. © 2015 Society of Plastics Engineers     

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.     

On the strength and stiffness of planar reinforced plastic resins

The recent history of planar reinforced plastic resins, including glass flake, high modulus ceramic flake, and continuous vapor coated film composites, is reviewed. The theoretical mechanics of both continuous (film) and discontinuous (flake and ribbon) reinforcements are summarized in simple form. A novel set of design curves is presented from which the lower bound requirements for the flake composite constitutents may be read directly. At the same time, the dependence of the composite ultimate strength on the shear strength of the plastic resin matrix is demonstrated. The mechanical properties of experimental film and flake composites representative of recent work are reported and compared with the theoretical predictions. In conclusion, the potential of planar reinforced plastic resin composites is discussed and found to be significant for applications where low weight and high isotropic stiffness are required, for example in aero-structural, airfoil, or blade components.