Glass transition temperature epoxy resin

Thermal mechanical analysis was utilized by Ophir 174) to study the densification of Bisphenol-A-based epoxies. The glass transition temperature can easily be characterized by a slope change as the resin transits from the glassy state to the rubbery state (see Fig. 25). Hence, in glassy material, it is typically represented by two thermal expansivity parameters, one below T (glassy thermal expansivity) and one... 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

The most noticeable property change is a decrease in the glass transition temperature of the epoxy resin as a function of absorbed dose. The decrease in Tg is due to plasticization by degradation products and free chain ends from chain scission. 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

Yamani and Young (5) applied the theory to explain the plastic deformation of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin cured with various amount of triethylene tetramine (TETA). They found that the theory gave a reasonable description for the resins below the glass transition temperatures T. ... 

Glass transition temperatures (Tg s) were detemined using a Dupont DSC 910 attached to a 9900 data analysis system. For off-stoichiometric studies, epoxy resin and diamine were cured in situ within a hermetically sealed DSC pan (sample tak from 25 C - 300 C at lO C/min), then cooled rapidly back to 25 C, and finally scanned from 40 C - 220 c to record the Tg. All samples were scanned under nitrogen atmosphere at a rate of 10 C/min. 

Figure lA. Glass transition temperature (Tg) and molecular weight between crosslinks (Me) as a function of epoxy/amine ratio for C-stage cured neat resin. 

The most widely used process for the fabrication of glass fabric prepreg involves solvent/solution techniques. The resins have to be soluble in appropriate solvents preferrably methylethylketone, acetone or other low boiling solvents. Therefore BMI building blocks with improved solubility are desired. Sometimes bismaleimides are used in epoxy resins to improve the glass transition temperature (T,), then solubility in epoxy resin is required. It has been found that tetraalkyl- 4,4 -MDA -BMIs(27), which melt at around 150°C and... 

In addition to the Bisphenol-A backbone epoxy resins, epoxies with substituted aromatic backbones and in the tri- and tetra- functional forms have been produced. Structure-property relationships exist so that an epoxy backbone chemistry can be selected for the desired end product property. Properties such as oxygen permeability, moisture vapor transmission and glass transition temperature have been related to the backbone structure of epoxy resins5). Whatever the backbone structure, resins containing only the pure monomeric form can be produced but usually a mixture of different molecular weight species are present with their distribution being dictated by the end-use of the resin. 

In blends of PTT and ABS, two separate glass transition temperatures are observed, which indicates that the blends are phase separated in the amorphous phase. A styrene/butadiene/maleic anhydride copolymer or glycidyl endcapped epoxy resin may act as a compatibilizer. Compatibilized PTT/ABS blends show a finer morphology and better adhesion between the phases. 

Glass transition temperatures were determined refractometrically, as described by Wiley (20), with an Abbe refractometer supplied by Bellingham and Stanley. This instrument can normally be used only at temperatures up to 70°C. The prisms of this instrument were coated with a heat resistant adhesive (epoxy-phenolic resin mixture). After this modification the refractometer can be used at temperatures up to 200°C. 

Recently, alternative theoretical expressions have been developed by using classical thermodynamic treatments to describe the compositional dependence of the glass transition temperature in miscible blends and further extended also to the epoxywater systems 2S,27). The studies carried out on DGEBA epoxy resins of relatively low glass transition have shown that the plasticization induced by water sorption can be described by theoretical predictions given by ... 

---