Polyimides are known to be useful high performance polymers. Polyetherimides are a particular class of polyimides which combine the high temperature characteristics of polyimides but still have sufficient melt processability to be easily formed by conventional molding techniques such as compression molding, gas assist molding, profile extrusion, thermoforming and injection molding. Similarly polysulfones are also known as useful engineering thermoplastics which generally have transparency, low color, resistance to hydrolysis, good melt processability and high temperature resistance. Methods are continually sought to prepare polymeric resins which combine the desirable properties of polyimides and polysulfones into a single resin, such as in a polyimide sulfone or a polyetherimide sulfone.
Good melt processability is a key attribute for both polyetherimides and polysulfones. It allows them to be quickly and easily formed into articles by extrusion and molding processes. However, good melt processability requires low levels of residual volatile species such as residual solvent and/or residual water, typically less than 500 ppm. Higher levels of residual volatile species generate bubbles and gassing which causes problems during melt processing of the resins into film and shaped parts by extrusion or injection molding processing. Such generation of volatile species from the molten resin can render articles made by a melt process unfit for use due to the presence of voids, bubbles, splay, silver streaks or other imperfections.
Another source of volatile species during melt processing is by reaction of the polymer, for example by reaction of the polymer end groups or cyclization of amide-acid groups remaining after the initial polymerization to make polyetherimides. These types of residual groups can react during melt processing to generate volatile species through condensation reactions. These volatile species can generate parts with similar defects as seen due to residual solvent. Thus polymeric resins which combine the desirable properties of polyimides and polysulfones must be made by a process that limits residual volatile species while giving low levels of residual end groups and amide-acid functionality.
U.S. Pat. No. 4,565,858 describes polyetherimide sulfones. However the importance of the full removal of solvent is not discussed nor is the level of residual groups. In certain examples the patent teaches isolation of the polymer by redissolving in chloroform, precipitation by addition to methanol and vacuum drying. This isolation method adds many extra steps to the process and generates a large amount of liquid waste, methanol and chloroform, which must be disposed of. The precipitation method described in this patent also gives a powder that requires another heat history before it can be made into conveniently used pellet form. A pellet form is desirable because pellets can be used directly in melt processing equipment such as extruders and molding machines.
While U.S. Pat. No. 4,565,858 does discuss end capping the resin with phthalic anhydride, no mention is made of the need to reduce the level of reactive groups to achieve optimal performance. High levels of end groups may react with each other or other additives in the resin mixture during subsequent melt processing generating water and changing the melt viscosity making melt processing difficult to control due to the changing nature of the polymer viscosity and the escape of water generated from the molten resin. Escaping water can also cause splay and silver streaking in molded parts. There remains a need for a method to prepare polymeric resins which have the desirable properties of polyimides and polysulfones in a single resin, such as in a polyetherimide sulfone, wherein said resins have a low level of residual volatile species such as residual solvent and a low level of potentially reactive groups which may generate volatile species during thermal processing.