Shelf-life polyimides

In light of the higher imidization temperatures associated with poly(amic alkyl esters), this approach could be extended to yield improved coating formulations [82]. Since it was now possible to balance the relative reactivity of the chain-extender with the imidization temperature of an oligofamic alkyl ester), here R = methyl, ethyl, etc., less reactive chain-extenders could be utilized. The lower reactivity of the chain-extender would be reflected in improved solution stability and shelf-life of the formulation without sacrificing the mechanical properties of the final polyimide, see Table 10 where EGX, TFE,... 

Some of the problems seen with the commercially available polyimides such as limited shelf life,gelation and high ionic contamination are traceable to the raw materials themselves. A zone refining technique has been perfected for use with organic materials and these precursors have been used to synthesize ultrapure polyamic acids for IC device applications. The key feature of the synthesis is the use of solid ingots of the dianhydrides. Materials prepared by this technique show low metallic impurities and have been shown to be excellent film formers for a variety of applications. In particular a polyimide derived from PMDA-ODA has been used to passivate magnetic bubble devices. IR techniques coupled with electrical measurements have been used to optimize the cure conditions and a simple resist process has been defined to passivate these devices. Device performance compares well with conventional inorganic insulators. 

Such combinations often allow solubility in relatively non-polar solvents to be achieved. The shelf-life of a polyimide is also very dependent upon the constituent diamine and anhydride used. Polyimides derived from bis(4-amino phenyl)thioether are stable for several months in boiling water whilst those derived from metaphenylene and paraphenylene diamines rapidly become brittle under the same conditions. Some typically used amines are... 

According to their chemical composition, conductive adhesive formulations have to he preserved either at low temperature, typically - 40°C for one-part epoxies, or at room temperature. For example, IP 680 polyimide or two-part epoxies can he kept at 20°C for at least 6 months, whereas one-part epoxies have a shelf life of 2 months at - 10°C and 6 months at - 40°C. The minimum shelf life requirement is 6 months at the temperature indicated hy the supplier. Material inspection includes the measurement of the pot life, corrosivity, volume resistivity at 25°C, and lap shear strength at 25°C. Typically pot life, bond strength, and volume resistivity have been used by adhesive manufacturers to determine the shelf life of the formulated materials. However, other methods such as DSC can predict the remaining shelf life by a quantitative measurement of the heat of reaction [58],... 

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