Preparation of polyimide foam

Details Preparation of polyimide foam Preparation of polyimide foam

1998-09-11

2001-01-09

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S090000, C521S117000, C521S128000, C521S185000, C528S183000, C528S209000

Reexamination Certificate

active

06172127

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention herein relates to a process of preparing a novel polyimide foam having superior heat-resistance, flame retardant, homogeneous size and distribution of cells, and low density, wherein a polyimide precursor in a granular form is prepared by means of using heterocyclic amine as catalyst and then foaming.
2. Description of the Prior Art
In general, the polymer foam refers to a light substrate in which the cells within the polymer are well distributed with low density. The foam is widely used as basic material in adiabatic, cushion, shock absorbent, soundproof material, and packaging material due to its superior cushion, adiabatic, soundproof, vibration-proof properties. The general purpose cells such as polyurethane or polyolefin cells are widely used in automobile, ships, and transportation equipment due to their superior strength, restoring capacity, adiabatic property, preparatory and economic efficiency. However, for a use in aerospace, submarine, special ships and high-speed train, which require superior heat-resistance and flame retardant, a special heat-resistance foam with superior heat-resistance and flame retardancy properties is used therein. With an aromatic or heterocyclic structure, a typical heat-resistance foam includes polyimide with high heat-resistance. In particular, the polyimide foam is known to have a wide application due to its superior thermal stability and flame retardancy.
The preparing process of a polyimide-based foam includes carrying out a foaming by means of heating a polyimide in an oligomer condition in an oven or microwave oven (U.S. Pat. No. 4,241,114; 4,241,193; 4,273,886; 4,296,208; 4,305,796; 4,332,656). However, the problem of a difficulty in control of the physical property arose in which a chain extension reaction and foaming occur simultaneously. Further, in U.S. Pat. No. 4,319,000, 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride (BTDA) and ethyl alcohol as esterifying agent were used to control the closed cell amount to the level below 95%. The polyimide foam was prepared by using two types of amines with a base dissociation constant of below 10
−10
, which were selected from the group consisting of 4,4-diaminodiphenylsulfone, 3,3′-diaminodiphenylsulfone, and 2,6-diaminopyridine. In order to obtain an polyimide foam with superior flexibility and homogeneous foaming structure, U.S. Pat. No. 4,369,261 used 3,3′, 4,4′-benzophenonetetracarboxylic acid dianhydride, methylenedianiline (MDA), 2,6-diaminopyridine, and activated carbon or graphite with superior conductivity. In U.S. Pat. Nos. 4,647,597, 4,656,198 and 4,670,473, a polyimide foam was prepared by using 3,3′, 4,4′-benzophenonetetracarboxylic acid dianhydride, methylenedianiline, and 2-methoxyethanol as plasticizer.
In U.S. Pat. Nos. 4,806,573, 4,824,874, and 4,830,883, 3,3′, 4,4′-benzophenonetetracarboxylic acid dianhydride, organic polyisocyanurate, and furfuryl alcohol were used to prepare a polyimide precursor, which in turn was used as a reenforcing agent of a foam with an open foam structure.
In U.S. Pat. No. 4,952,611, a polyimide foam, with superior compression set property for a use in seat cushion, was prepared by using a mixture of 3,3′, 4,4′-benzophenonetetracarboxylic acid dianhydride and 2-(vicynal-dicarboxycyclohexenyl)-succinic acid in addition to an amine selected from methylenedianiline or 2,6-diaminopyridine. diaminopyridine.
In order to obtain a cell size of 0.1 to 1.0 mm in a polyimide foam, in U.S. Pat. No. 4,978,692, an amine was selected form the group consisting of 3,3′, 4,4′-benzophenonetetracarboxylic acid dianhydride, 4,4-bis[4-(3-aminophenoxy)phenyl]sulfide, and 4,4-bis[4-(3-aminophenoxy)phenyl]sulfone, 4,4-bis[4-(3-aminophenoxy)vinyl]benzophenon.
In U.S. Pat. No. 5,234,966, for preparing a polyimide foam of a target density, 3,3′, 4,4′-benzophenonetetracarboxylic acid dianhydride, methylenedianiline, and 2,6-diaminopyridine was used, and a mixture of alcohol and water were used as esterifying agent.
SUMMARY OF THE INVENTION
In the present invention, an aromatic carboxylic acid or the anhydrides thereof, and an aromatic or heterocyclic amine were used to prepare a polyimide precursor. In such process, heterocyclic amine, which has not been known to be applicable herein, was used as catalyst in order to prepare a polyimide precursor. By means of using the aforementioned precursor, a novel polyimide foam which has low density, homogeneous cell size and distribution, superior restoring capacity and flame retardancy was prepared.
The objective of the invention lies in providing a preparing process of a polyimide foam with low density, homogeneous cell size and distribution, superior restoring capacity and flame retardancy by means of using a polyimide precursor which was prepared by using heterocyclic amine was catalyst.
In order to achieve the objective according to the present invention, the preparing method of a polyimide foam comprises reacting aromatic carboxylic acid or the anhydrides thereof with an excess of aliphatic univalent alcohol to yield an aromatic ester solution. To the aromatic ester solution, divalent amines or the mixture thereof were added in the equivalent amount of said carboxylic acid or the anhydrides thereof in addition to a catalyst and surfactant to yield a polyimide. Then, the precursor in a granular form mixture was imidized while foaming by means of pre-heating and then heating in a microwave oven, after which was cured at a high temperature.
DETAILED DESCRIPTION OF THE INVENTION
The polyimide foam according to the present invention is prepared as follows: The following compounds were added to yield a polyimide precursor in solution: 1 mole of aromatic carboxylic acid or the anhydrides thereof; 1 mole of divalent aromatic amine or the alicyclic divalent amine; 6 to 20 moles of aliphatic univalent alcohol with 1 or 6 carbon atoms as esterifying agent; 0.01 to 10 mole % of heterocyclic amine catalyst to said aromatic carboxyl acid or the anhydrides thereof; and 0.02 to 2 wgt % of surfactant to the total amount of said aromatic carboxylic acid or the anhydrides thereof and said divalent amine. Then, the powder in solid phase was prepared by removing alcohol which was used as esterifying agent at 50° C. in the rotary evaporator. The particle size of said powder was controlled to 300 &mgr;m or less by using a sieve. Thereafter, it was placed in an oven with reduced pressure at a temperature range of 40 to 60° C. to completely remove alcohol therefrom. The polyimide precursor showed the intrinsic viscosity of 0.1 dL/g to 0.2 dL/g measured at 30° C. by using dimethylacetamide as solvent.
After placing the completely dried polyimide precursor onto the Teflon sheet, it was pre-heated for 1 to 30 minutes at 120° C. to 180° C. in a convection oven. Thereafter, the polyimide agent was prepared with an imidization level of 10 to 50% by controlling the on/off time on the microwave oven with the frequency of 2,450 MHz. The polyimide foam so prepared, however, was easily breakable due to its condition of simple foaming. Consequently, the polyimide was cured in an oven with a heat fan at 200~300° C. for 0.5 to 4 hours, thereby resulting in 70% or more of opened cell structure. By using the above process, a polyimide foam was prepared having superior restoring capacity and flame retardancy with the average cell size of 50 &mgr;m to 2 mm, and the density of 3 to 25 kg/m
3
.
According to the preparing process of a polyimide foam, if the amount of aliphatic alcohol, which is used as esterifying agent of aromatic carboxylic acid or the anhydrides thereof, is less than 6 mole or more than 20 mole, the esterification reaction of aromatic carboxylic acid or the anhydrides thereof is carried out at a unduly slow rate, and a complete esterification is not achieved. If the amount of catalyst during the prepare of the polyimide precursor is less than 0.01 mol

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