The present invention generally relates to a new method for preparing high molecular weight, easily processed, fully-cyclized polyimides containing controlled amounts of aliphatic groups making polyimides.
Polyimides are useful in a variety of applications. Polyimides may be used in flexible printed circuit boards for electronic devices. Other applications include, adhesives, matrix resins for composites, as well as films and coatings.
Typically, polyimides are made by the condensation of a diamine and a dianhydride. This reaction initially results in the formation of a prepolymer, polyamic acid, followed by heating the polyamic acid to form the polyimide. However, the use of aliphatic diamines poses difficulties in generating high molecular weight polyimides because of complexation (salt formation) after reaction with conventional dianhydrides. Typically, salt complexes are formed and gelation of the reaction mixture is often observed. Further, it is difficult to control the amount of aliphatic units in the polyimide when the poly amic acid route is utilized.
It is an object of the present invention to provide a method for making a polyimide where the polyimide contains a controlled amount of aliphatic units.
Further, it is an object of the present invention to provide a method that reduces the formation of salt-like complexes and gelation that is associated with the polyamic acid route.
The invention also includes the polyimide material made by the process of the present invention.
Accordingly, it has been found that a high molecular weight polyimide that is fully cyclized and contains a controlled amount of aliphatic units can be prepared by converting the dianhydride to an ester acid and reacting the ester acid with a controlled amount of an aliphatic diamine.
The invention includes reacting a diamine with an ester acid of a dianhydride to form a polyimide. A wide range of dianhydrides could be used in this invention. For example, the dianhydride may be pyromellitic dianhydride, bisphenol A dianhydride, or combinations thereof. The diamine may include aliphatic diamines, aromatic diamines, and combinations thereof. For example, the diamine may be dodecyldiamine, oxydianiline, diaminophenylindane, hexamethylene diamine, cyclohexyl diamine, and combinations thereof. Preferably, the ester acid is formed by reacting the dianhydride with an alcohol to form the ester acid.
Still further, the present invention includes a method for making a polyimide comprising the steps of converting a dianhydride to an ester acid and reacting a predetermined amount of an aliphatic diamine with the ester acid. The step of reacting a predetermined amount of an aliphatic diamine with the ester acid may include reacting a predetermined amount of a second diamine with the ester acid. The second diamine is preferably aromatic. In this way, a blend is produced where the resulting polyimide includes controlled amounts of aliphatic and aromatic monomers.