Glass, transition shelf life

Table 12.8 shows the effect of three commercially available substituted ureas on shelf life, cure rate, exotherm, and glass transition temperature of a dicyandiamide cured epoxy adhesive. The accelerators are compared at use levels of 1, 3, and 5 pph in a one-component adhesive consisting of 100 pph of DGEBA epoxy, 8 pph of dicyandiamide, and 3 pph of... 

Radiant cured epoxy systems also provide high glass transition temperatures, low shrinkage during cure, and relatively low residual stresses. They have no volatile emissions and excellent shelf life. The good physical and adhesive properties of conventional epoxy adhesives systems are maintained with the radiant cured systems. 

Roos, Y.H., Karel, M., and Kokini, J.L. (1996). Glass transitions in low moisture and frozen foods effeets on shelf life and quality. FoodTechnol, 50(11), 95-108. 

Edmond, J. The Effect of Ingredients on the Glass Transition Temperature and the Shelf Life of Marshmallow, Honours thesis, University of Otago, New Zealand, 2000. 

For all materials the amorphous form is thermodynamically unstable. Any unstable system has to have a mechanism by which to transfer to its stable (lowest-energy) state. The activation barrier that needs to be passed to move to the stable state will determine whether the transition is spontaneous. The transition from the amorphous to the crystalline form will depend on the mobility of the molecules. When below the glass transition temperature (Tg), the molecules lack sufficient mobility to allow spontaneous crystallization (at least within the time scale of observation of in situ experiments). However, this does not mean that materials do not crystallize below Tg, it means that there is an issue of the duration over which the system is observed and the activation energy needed to allow crystallization to occur. For example, Yoshioka et al. (18) reported crystallization of indomethacin within a few weeks when stored at 30°C below Tg, but stability of in excess of a year when stored at about 50°C below Tg. Hancock et al. (19) calculated that for all significant (i.e., capable of affecting a shelf life) mobility to cease, indomethacin needed to be stored at a minimum temperature of 50°C below Tg. 

Control of the spontaneous condensation of the silanols formed during and after the hydrolysis reaction is very important. If too many silanols condense during hydrolysis, the shelf life will be very short and gel formation will readily occur. Therefore, the hydrolysis of the alkoxysilane is carried out under mild conditions and the resulting partial condensate is diluted with water soluble solvents to a lower solids level. The resin is cured by further silanol condensation which can occur with heat alone. This, however, is a relatively slow process that requires cure temperatures above the glass transition temperature (Tg) of the plastic substrate. While many catalysts can be used, including acids. 

Adhesi V6 Matrix. Polymer matrices of ICAs are similar to ACAs. An ideal matrix for ICAs should exhibit a long shelf life (good room temperature latency), fast cure, relatively high glass-transition temperature (Tg), low moisture pickup, and good adhesion (39). 

It will be useful to estimate how far from the glass transition temperature an amorphous material can be stored to ensure that the material is kinetically stable over the duration of shelf life. Several rules of thumb have been proposed, of which probably the best known is the Tg - 50 rule for estimating the required storage temperature (Hancock et al. 1995). If this temperature is far below room temperature, storage of the amorphous dmg at that temperature will not be practical, and alternative means of stabilizing the amorphous drug will be needed. (It should be noted that the Tg - 50 rule is a generalization based on concepts such as those detailed above. While many materials do not follow this rule, it is still a useful concept.)... 

Sometimes, the traditional methods (such as salt formation) used for increasing the dissolution rate and thereby the bioavailability of the drug in the body may not be feasible. This was the case for 2-[4-(4-chloro-2-fluorophenoxy)-phenyl]pyrimidine-4-carboxamide, a sodium channel blocker. It was a weak base with very low solubility of 0.1 pg mL in water, simulated gastric fluid (SGF) and simulated intestinal fluid (SIF). The extremely low pATa of —0.7, made it non-amenable to salt formation, even with strong acids such as hydrochloric acid, sulfuric acid and phosphoric acid. Also, its glass transition temperature (Jg) at 43 °C, was close to that at room temperature, which limited the formation of the amorphous phase reproducibly. It should also be noted that the amorphous phase, which might be an option in some cases, was not desirable either. This is because it could lead to the formation of a metastable state that could limit its shelf life. As a result, co-crystal formation was attempted in order to manipulate its physicochemical and pharmacokinetic properties. 

Thermal properties of adhesives affect their behavior in a number of diverse ways. Firstly cure processes, pot life and shelf life depend on temperature. High temperatures can limit the lives of adhesives by causing chemical degradation and in some cases by introducing stresses due to the differences in thermal expansion. Glass transition is the most important temperature for an adhesive, in that it demarcates rubbery adhesives from rigid ones, and is something to be avoided in service. 

---