Preparation of imide oligomers via concurrent reactive extrusion
Reactive extrusion can be used in a continuous, solvent-less preparation of imide oligomers involving two competing reactions among three ingredients, the first reaction between a dianhydride and a diamine and the second reaction between an endcap and the same diamine. The imide oligomer can form a composite via conventional production methods or via formation of a film from imide oligomer re-melted in an extruder before being impregnated into tape or fabric.
1. A process for preparing low-melt viscosity imide oligomers derived from a solvent-free reaction of stoichiometric effective amounts of at least one asymmetric dianhydride having the formula:
wherein X is selected from the group consisting of nil, C═O, —CH2 and oxygen,
and at least one aromatic diamine having the formula selected from the group consisting of:
wherein the Y radicals are either the same or different and are selected from the group consisting of nil, CH2, C2H4, C═O, and oxygen,
and an endcap selected from the group consisting of 4-phenylethynylphthalic anhydride and cis-5-norbornene-endo-2,3-dicarboxylic anhydride,
wherein the process comprises the steps of:
(a) introducing the asymmetric dianhydride, the aromatic diamine, and the endcap into an extruder at its throat;
(b) melt mixing the asymmetric dianhydride, the aromatic diamine, and the endcap for a sufficient period of residence time in at least one mixing zone having at least one temperature to thoroughly blend them together and to permit concurrent reaction of both the asymmetric dianhydride and the endcap with the diamine to form the imide oligomer;
(c) extruding the imide oligomer from the extruder;
(d) melt mixing the imide oligomer in the presence of carbon nanofiber to form a fiber reinforced imide oligomer compound having about 17 to about 23 weight percent of carbon nanofiber;
(e) extruding the fiber reinforced imide oligomer compound; and
(f) feeding the fiber reinforced imide oligomer compound into a machine for impregnation into a tape or fabric.
2. The process of claim 1 , wherein the temperature in the extruder ranges from about 210° to about 380° C. in an extruder of at least six zones.
3. The process of claim 1 , wherein the melt mixing step (b) has a residence time of about 20 to about 2000 seconds.
4. The process of claim 1 , wherein the melt mixing step (b) has a residence time of about 25 to about 200 seconds indexed to a 16 mm, L/D=40 extruder.
5. The process of claim 1 , wherein step (e) results in pelletization before step (f).
6. The process of claim 1 , wherein the endcap introduced during step (a) competes and denies reaction sites at the aromatic diamine for reaction by the dianhydride.
7. The process of claim 1 , wherein introduction of the endcap of the extruder during step (a) establishes competition for diamine reaction sites between the dianhydride and the endcap from the beginning of the extrusion process.
8. The process of claim 1 , wherein the melt mixing step (b) has a screw speed of from about 100 to about 1000 revolutions per minute.
9. The process of claim 8 , wherein the melt mixing step (b) has a screw speed of from about 100 to about 300 revolutions per minute indexed to a 16 mm, L/D=40 extruder.
10. The process of claim 1 , wherein the endcap is a solid or a liquid, and wherein the process is solvent-less.
11. The process of claim 1 , wherein the introduction of the endcap during step (a) does not cause immediate cessation of reaction of the dianhydride and the diamine which have also begun reacting during step (a).
12. The process of claim 1 , wherein the imide oligomer further comprises optional additives selected from the group consisting of 4-phenylethynyldiphenyl methane and diphenylacetylene, and combinations of them.
13. The process of claim 1 , wherein the tape or fabric comprises carbon fibers, fiberglass, polymers, or combinations of them.
14. A composite formed from the fiber reinforced imide oligomer compound made by the process of claim 1 .
15. The composite of claim 14 , wherein the fiber reinforced imide oligomer compound is further reacted or cured at temperatures ranging from about 340° to 360° C. to form a thermosetting polyimide matrix-composite having a Tg greater than 400° C.
16. The composite of claim 15 , wherein the carbon nanofiber is present in amount of about 18 to about 22 weight percent of the fiber reinforced imide oligomer compound.
17. The composite of claim 14 , wherein the composite is in the form of an article selected from the group consisting of an airframe component; an engine component; an aircraft; a missile; a radome; a rocket, a high temperature laminate; an electrical transformer; an engine bushing; an engine bearing; oil drilling equipment; oil drilling risers; automotive chassis bearings; and films for use in electronics, fuel cells and batteries.