Temperature effect, epoxy curing

On the other hand, if the cure rate is much faster than the phase separation, then the morphology is controlled by the cure rate through a chemical pinning process. In this system, phase separation is mainly controlled by the cure rate of the epoxy matrix. Faster curing rates and shorter gel times lead to smaller PEI-rich particles with an increasing cure temperature. The temperature effect on the viscosity of reaction mixture is relatively small (i.e., the complex viscosities measured by Physica are 7 and 4 Pa.s at curing temperatures of 150 and 190°C, respectively). 

In the composition range where the nodular morphology was shown, because the matrix (PEI phase) contained relatively small amount of epoxy, the temperature effect on the viscosity was larger than the reaction effect. Thus, the viscosity of the PEI matrix in the nodular morphology became reduced as the cure temperature was increased, which made epoxy nodules coalesce more easily with each other. As the cure temperature is increased, the viscosity of the PEI-rich matrix decreases from 210 Pa.s at 150°C to 50 Pa.s at 190°C. 

These aliphatic amines can also be cured at elevated temperatures to provide a more densely crosslinked structure with better mechanical properties, elevated-temperature performance, and chemical resistance. Table 5.3 illustrates the effect of curing temperature on the bond strength of DGEB A epoxy with two different aliphatic amines. 

TABLE 5.3 Effect of Curing Temperature on Bond Strength of DGEBA Epoxy Resin Cured with Two Different Aliphatic Polyamines6... 

It has also been shown in several studies that the combination of stress, temperature, and moisture can accelerate the hydrolytic instability of certain epoxy adhesives. In an FTIR study of the effect of moisture on DGEBA epoxy cured with nadic methyl anhydride, spectra changes were observed in stressed specimens aged for 155 days at 80°C and 100 percent RH.47 This was attributed to the slow, stress-induced hydrolysis of ester groups. In another study,... 

FIGURE 15.20 Effect of outdoor weathering on typical aluminum joints made with four different two-part epoxies cured at room temperature.65... 

Hydantoin epoxy resins having glycidyl groups in the 1- and 3-positions and one or two alkyl groups in the 5-position were prepared by the Bucherer reaction, followed by treatment with epichlorohydrin. These resins were crosslinked with hexahydrophthalic anhydride to examine the effect of alkyl substituents on the glass transition temperatures of the cured systems. 

Gel Tg. the temperature at which gelation and vitrification occm simultaneously Tgo. the Tg of the mixed reactants, corresponding to a minimiun cure In isothermal cures, at temperatures between the gel Tg and Tgo, the resin undergoes gelation followed by vitrification. Since Tg is a function of both the degree of cure and the cross-link density, it increases to a point at which aU polymer reaction sites have been consumed. At that point, the Tg reaches a plateau and does not increase further with temperature until the decomposition temperature where a char region occurs (Fig. 2.17). An example of the effect of cure conditions on Tg and CTE values was shown by Konarski " who varied the cure cycles of an anhydride-cured epoxy from 130 to 175 °C as shown in Table 2.10. The value of Tg continued to... 

Both polysulfone and the polyetherimide are thermojdaslics whieh withstand higher temperatures than epoxies such as DGEBA. The effect of (43) a polysulfone (Udel, 43a) and polyetherimide (Ultem, 43b) thermoplastics on the properties of a tetra-2,3-epoxypropylamine, N,N,N, N -tetra-2,3-epoxypropyl-, -bis(4-aminophenyl)-p-diiso-propenylbenzene (HPT Resin 1071,43c) has been studied and also the effect of these thermoplastics on the resin di-2,3-epoxypropyl-9,9-bis (hydro phenyl)fluorene (HPT Resin 1079,43d). Both these resins have a stiff backbone and were developed for use as amatrix resin for advanced composites. They can be cured with the aromatic dianune, -bis(3,5-dimethyl-4-aminophenyl)-p-diisopropylbenzene (43e). These resin curing agent combinations exhibit greatly improved hot/wet performaiKe over that of traditional epoxies. 

Fig. 2.9. Influence of temperature and filler content on flow curves (Ref. 14). (a) Temperature effect on 1-part hot cure toughened epoxy. (i>) Filler content effect on 3-part flexibilised epoxy polyamine.

Rate of strength development. The effect of temperature on curing rate will vary for different adhesives. In general, low temperatures increase the curing period considerably and many epoxy resin formulations stop curing altogether below 5 °C. A rule of thumb often quoted is that the curing period doubles for every 10 °C fall in temperature below ambient but halves for every 10 °C rise in temperature above ambient. 

The DSC curves for thermoset epoxy with different weight fraction of organo-modified nano-clay are shown in Figure 9.11. The onset temperature of the curing and the temperature of the exothermal peak for neat resin are 115°C and 155 C, respectively. The addition of 5 wt% nano-clay in epoxy matrix reduces the onset temperature to 84°C and peak exothermal temperature to 143°C. The catalytic effect of the nano-clay on the crosslinking reaction of epoxy resin is responsible for the reduction. 

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