Paul Peyser

The anomalous lowering of the glass transition of an epoxy resin by plasticization with water

The validity of the assertion that 6 to 7 wt% absorbed water lowers the glass transition,Tg, (~250° C) of a highly cross-linked, high temperature epoxy resin (NARMCO 5208) by 100 to 150° C was investigated by a number of thermoanalytical techniques. This plasticization is of significance to the use of organic matrix composition skins on supersonic aircraft which experience a sudden “thermal spike”, and supposedly a sudden loss in modulus. The study was complicated by the loss of water above 100° C during the tests and the decomposition of the polymer at about the temperature of its apparent glass transition. No glass transition could be observed by differential scanning calorimetry and the thermal mechanical analyser gave inconclusive results. The dynamic mechanical analyser gave a clear indication of a glass-like transition at about 250° C which was reversible, reappearing upon cooling and reheating. This transition could not be related to the decomposition of the sample. Absorption of 7 wt% water broadened and lowered the transition by about 50° C. However, after studying the rate of change of the modulus of the resin from the plasticized state to the dry state it was concluded that the absorbed water lowersTg by only 50° C and not the 100 to 150° C claimed by others. Moreover, the mechanism of this apparent glass transition differs from that of a normalTg, and may involve the breaking of hydrogen bonds.

Kinetics of an Anhydride-Epoxy Polymerization as Determined by Differential Scanning Calorimetry

The use of differential scanning calorimetry (DSC) to study the thermal behavior of polymers has developed to the point where it is now used to establish the kinetics of polymer phase transformations, including polymerization reactions. We will describe the use of DSC to determine the polymerization kinetics of the anhydride-epoxy system, bisphenol A diglycidyl ether (DGEBA)-”nadic” methyl anhydride (NMA). This polymerization reaction, catalyzed by benzyldimethyl amine (BDMA) is generally viewed as a very complex curing process but the data here were fitted to a two stage model utilizing a single reaction order of n=2 and (only) two activation energies. Namely, in dynamic scans, after the reaction had proceeded with an activation of about 15 kcal/mole for about 12% completion, the rate increased until completion but with a higher activation energy of about 38 kcal/mole. Such an increase in rate was verified qualitatively by an isothermal run. Other workers studying similar systems have reported the lower activation energy. However, scrutiny of their data indicates a similar increase in rate.

Effect of filler and cooling rate on the glass transition of polymers

Abstract Fillers have been reported to raise, have no effect upon, or to lower the glass transition temperature Tgof polymers. In those studies, comparisons have been made between filled and unfilled polymers having equal thermal histories. In the work report here, however, the thermal history (cooling rate) was also varied. Two systems, polystyrene-silica and epoxy-rubber were studied. The glass transitions were measured by using a differential scanning calorimeter and a method of analysis first described by Ellerstein and later elaborated upon by Flynn. The data could be represented by a linear plot of the log of cooling rate versus Tgor Tg −1. For the polystyrene system, the filled material had a larger negative slope for the plot of log cooling rate versus Tg −1 so that after fast cooling the filled material had a lower Tg than the unfilled material. However, when the cooling rate was lowered, the Tg of the filled and unfilled materials approached each other, and for very slow cooling (annealed sample...

An evaluation of the short rod technique to measure the fracture toughness of polymers

The investigation of the fundamental variables which influence the fracture toughness of structural plastics is greatly hampered by a large amount of scatter and uncertainty associated with the fracture toughness measurement. A major part of the problem is due to a lack of adherence to ASTM Standard E399, mainly with regard to the requirement for a fatigue crack. A razor-blade arrested crack, which is often blunted, is common practice in the plastics field. It is also common to ignore size (plane strain) and precise machining requirements. The short rod (SR) method was evaluated as a potentially more precise and simpler fracture toughness measurement. This toughness measurement is made on a slowly moving and presumably sharp crack, and the geometry of the sample enforces plane strain conditions. Toughness measurements on compact tension (CT) specimens via ASTM E399 were performed on one-inch (25 mm) samples of poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and polysulphone (PSO). Also, a constant compliance method using a contoured double cantilever beam (CDCB) was used to evaluate the toughness of PS, PC, and PSO, but in general we did not achieve stable crack growth. The used samples were then fabricated into SR specimens and their toughness measured. The CT and CDCB methods agreed with each other for PSO and PC, but for PS the CDCB method gave high values. It is argued that the SR method should be compared to the other methods without using a plasticity correction. Then the SR method agrees well with the CT method for PSO and PS and is 15% higher for PC. The PMMA SR results were invalid. Differences between the methods are explained in terms of crack blunting, rate effects, non-homogeneity, residual stresses and the global nature of the crack front. The SR method has promise for polymer evaluation but more experience and evaluation is needed. The method is unique in the ability to study the effects of thermal history and of the environment on fracture toughness.

The Fracture Behavior of CTBN Modified Epoxy

Abstract Rubber toughened epoxy resins are widely used as adhesives, as a matrix for glass and carbon fiber composites for rocket cases and sporting goods, and as a potting agent in the electronics industry. A common rubber added is CTBN, a carboxyl terminated copolymer of butadiene and acrylonitrile. In this study we have measured toughness (K1C and G1C ) using the E399 ASTM standard for a compact tension (CT) specimen, with special attention to the variability of the measured K1C and G1C with the method of starter crack formation and the time delay between starter crack formation and toughness measurement. We also investigated the toughness of the toughened epoxy after initiation, for a growing crack, by using the short rod (SR) method and when possible in a CT specimen by using a simple marking technique. The CT toughness of unmodified epoxy measured using a liquid nitrogen initiator crack technique is the same as that of earlier work, but we found that the KIC and G1C toughness increases when there is...

A new method for the determination of the kinetics of Tg

Abstract Differential scanning calorimetry has been found useful in studying the kinetics of the Tg. The method commonly used involves annealing the polymer at temperatures below Tg for various periods of time and then measuring the enthalpy change in going from the glassy to the liquid state as a function of annealing time and temperature. However, annealing below Tg is a slow process, and therefore the quantity of research that can be done by this method is limited. Moreover, the above method measures primarily the kinetics of the annealing process. We are also interested in the kinetics of the transition from the glass to the liquid state. We have developed a DSC method to measure the kinetics of the transition of a polymer from the glassy to the liquid state. It involves quickly heating the polymer from below the glass transition to slightly above it and measuring the change in enthalpy with time as the polymer goes from the glass to the liquid. The method depends on subtracting out, by use of a compu...