Process for Producing Articles From Polymers Containing Aromatic Imide Linkages

1. Field of the Invention

This invention relates to polymers containing aromatic imide linkages, commonly referred to as polyimides (PI). Specifically, the invention relates to the production of articles made from polyimides in shorter times and by simpler methods than has been previously possible.

2. Description of the Related Art

Polyimides are classified as condensation polymers produced from reaction of bifunctional diamines and bifunctional or tetrafunctional carboxylic acid anhydrides. The imide structure --CO--NR--CO is found in the backbone of the polymer chain. Aromatic, heterocyclic imides are known for their exceptional mechanical characteristics and excellent chemical and high temperature resistances. Many types of aromatic polyimides have been created, depending on the amine and acid chemical structures. Pyromellitic dianhydride and di-(4-aminophenyl) ether yields Dupont's commercial polyimide product, Vespel.TM.. Trimellitic anhydride and an amine such as o-bisaniline or trimellitoyl chloride and methylene dianiline yields Amoco's commercial polyamide-imide product, Torlon.TM.. The self-condensation of trimellitic acid isocyanate yields Upjohn's P2800.TM.. Condensation of bisphenol and dinitrobisimide yields General Electric's commercial polyetherimide product, Ultem.TM.. Bismaleimides are produced by reacting maleic anhydride with diamines. Ciba-Geigy's P13N.TM. and Rhone-Poulenc's Kinel.TM. are commercial examples of reactive bismaleimides. Aromatic polyester, anhydrides and certain bismides yield polyesterimides, such as Dynamit Nobel's Icdal Ti40.TM.. Aliphatic polyester anhydrides yield polyarylates. Other pure polyimides are Lenzing P84-HCM.TM., Mitsui-Toatsu Aurum .TM., Furon Meldin 3000.TM., National Starch Thermid.TM. and NASA Langley LaRC TPI series.

Polyimides are generally characterized by great backbone rigidity that yields exceptional high temperature strength and thermal stability. They are frequently used for high temperature applications [>230.degree. C.]. As a result, they are used as bearing seals, gaskets, piston rings, pressure discs, sleeves, sliding rods, valve shafts, automotive and appliance gears, brake components, cams, exhaust valves and stems, copier gears and so on. They are also characterized by extreme processing difficulty. Many polyimides cannot be molded by conventional thermoplastic processing techniques. Others require very high processing temperatures [300.degree. C. to 400.degree. C. or more] and pressures [1000 atmospheres or more]. As a result, polyimides are usually processed as powders using powder metallurgical techniques such as hot isostatic pressing, direct forming and compaction-sinter-forging or by compression molding where the powder is placed in a heated mold, heated to a forming temperature, compressed into shape, then cooled in the mold while under pressure. Many parts are machined from billets. Alternately, the polymers are dissolved in a suitable solvent such as n-methylpyrrolidone. Small molded parts, thin films and carbon-fiber laminates are produced from these solutions. With polyamide-imide polymer, the polymer is injection molded or extruded while it is not fully imidized. The molded or extruded part is then reacted at elevated temperature but in the solid state to a fully imidized state. Fully imidized polyamideimide cannot be molded using conventional plastic process techniques. It is believed that amorphous polyimides require decomposition temperatures higher than their melt processing temperatures and that the melt processing temperatures must be at least 50.degree. C. to 100.degree. C. above the glass transition temperatures of the polymers.

The limited tractability of polyimides with conventional plastics processing equipment has restricted the development of many applications. It has now been found that polyimides, including reactive polyamide-imides, can be molded into articles of commerce by first rapidly heating the polymer powder without shear to proper molding temperature, then compressing the powder into the desired shapes using conventional compression molding techniques. In a similar fashion, polyimide powder can be first rapidly heated without shear to a proper molding temperature, then ram extruded into desired profiles. The method of rapid heating involves mechanically impacting and imparting kinetic energy to the powder particles in a batch-wise fashion, while restricting and controlling the transfer of energy from the particles to the chamber holding the powder particles. One device that is used as a powder heater is shown by Goeser et al, U.S. Pat. No. 3,266,738, published Aug. 16, 1966. This patent describes a high intensity mixer available on the market today, under the trade mark Gelimat, made by Draiswerke GmbH. The device includes a plurality of blades that rotate about a horizontal axis within an enclosed jacketed container. The rotational speed of the blades is controlled with tip speeds of up to 50 meters/second possible. In Crocker et al. U.S. Pat. No. Re. 33,214, this device is coupled with an infrared detector that continuously monitors the powder temperature to ensure adequate temperature control while the constant rotational speed of the blades exceeds 25 meters/second. In Crocker et al. U.S. Pat. No. 4,420,449, published Dec. 13, 1983, the device so equipped is used to thermokinetically heat polytetrafluoroethylene (PTFE) powder at constant tip speeds of at least 30 meters/second prior to forming. In Crocker U.S. Pat. No. 4,272,474, published Jun. 9, 1981, the device so equipped is used to thermokinetically heat ultrahigh molecular weight polyethylene (UHMWPE) powder at constant tip speeds of at least 18 meters/second prior to forming.

It has surprisingly been found that it is not possible to mechanically heat polyimide powders only using either continuous monitoring of powder temperature or constant tip speeds of 18 meters/second, 25 meters/second, or 30 meters/second or any other constant tip speeds. It has further surprisingly been found that the heating rate of polyimide powders can be altered over wide ranges with careful balance of the chamber wall temperature and the speed of the mechanical heating device. This control allows for uniform heating of powder charges of widely varying rates. It further allows for controlled heating of powder charges such that partially imidized powder can be solid state reacted to full imidization at temperatures a few degrees below the molding temperatures. It further allows for degassing, devolatilizing and dewatering of polyimides that might contain byproducts of the condensation polymerization process including dissolved gases, volatile liquids, water, acetic acid, hydrochloric acid or other low boiling simple molecules. And it is further surprisingly been found that certain polyimides that are known to be unmoldable by any conventional plastic processing technique, can be mechanically heated to very high temperatures, typically in excess of their degradation temperature using a mechanical heating device with very hot chamber wall temperatures, in excess of 150.degree. C., and that these polyimides, when discharged from the mechanical heating device, can be molded into useful monolithic articles using conventional compression molding techniques with molds having temperatures 10.degree. C. to as much as 100.degree. C. below the glass transition temperatures of the polyimide. And it is further surprisingly been found that the reground powders of certain polyimides known to have a certain degree of crosslinking, to be considered as having thermosetting characteristics and to otherwise be considered as having no reprocessing capability, can be mechanically heated in a mechanical heating device with very hot chamber walls and the discharge molded into useful monolithic articles using conventional compression molding techniques.

After the heated polyimide powder is discharged from the mechanical heating device, the charge is transferred to a mold or die contained in conventional plastics processing equipment such as a compression molding press, a transfer molding press, a ram extruder, a forging press or other hydraulically, pneumatically or mechanically assisted press whereupon pressure is applied that is adequate to force the charge into the desired shape in the mold or die that has a temperature about 10.degree. C. to 100.degree. C. or more below the glass transition temperature of the polymer. The pressure is held against the shaped charge until the charge retains the general shape of the mold cavity. Articles made in this fashion have essentially no porosity, essentially no internal voids, essentially no cracks and mechanical properties that are equal to or greater than those of polyimides fabricated in more traditional ways.

The present invention provides a process for producing an article from at least one polyimide polymer, the process consisting of setting the mechanical heating device chamber wall temperature, charging the device with a carefully measured amount of polyimide polymer powder, powering the impellers to heat the polymer powder for a specific period of time, reducing the impeller speed to a point where energy input from the impellers to the agitated powder just matches energy transfer from the agitated powder to the chamber walls, maintaining that state for a specific period of time to allow for solid state reaction, devolatilization, dewatering, degassing or other thermal conditioning to occur in the polyimide polymer powder, increasing impeller speed for a specified period of time, discharging the heated powder, transferring the powder charge to an appropriate conventional processing device and applying necessary pressure at specific mold or die temperature while the charge cools, to insure adequate formation of at least one desired monolithic article of commerce.

Although each polyimide polymer requires specific processing sequence, experience shows that a typical charge weight is 50 g to 250 g per liter volume of the mechanical heating device. Differences in charge weights must be compensated for by adjusting impeller speeds and the wall temperature of the mechanical heating device chamber. Heating rates of 2.degree. C. to 10.degree. C./second are desired during initial heating when the mechanical heating device wall chambers are approximately 100.degree. C. below to 50.degree. C. above the polymer glass transition temperature. The desired period of time when the heat input to the powder from the impellers just matches the heat output from the powder to the chamber walls is about 30 seconds but could be in the range of 1 to 60 seconds. The powder temperature during this period of time is above the glass transition temperature of the polyimide and preferably less than 150.degree. C. above the glass transition temperature of the polyimide. Under no circumstances is the temperature to be equal to or greater than the temperature where the powder particles agglomerate. Experience shows that the heating rate subsequent to this time is 50.degree. C. to 80.degree. C./second, with polymer powder discharge temperatures of 50.degree. C. to perhaps 150.degree. C. above the glass transition temperature of the polymer. The total heating time for charges ranging in weights from 50 g to 250 g per liter volume of the mechanical heating device ranges from 15 seconds to 75 seconds and depends on the time required to react, devolatilize, dewater, degas, or otherwise thermally condition the powder below its glass transition temperature. As noted, thermal processing of the hot powder occurs with molds or dies at temperatures about 10.degree. C. to 100.degree. C. or more below the glass transition temperature of the polymer. Molding pressures are considered to be normal for the processing equipment, with compression molding and extrusion pressures on the order of 1,000 lb.sub.f /in.sup.2 [7 MPa]. Actual values depend on the nature of the polyimide being processed.

Certain polyimide polymers are commercially available and were noted earlier. Other polyimide polymers are experimental and so exact processing conditions have not been established.

The Draiswerke Gelimat used for mechanically heating polyimides had a 1-liter volume chamber capacity. The Gelimat has opposed impellers mounted on an axial shaft and driven by a variable speed motor that turns the shaft at up to 7000 revolutions/minute. The powder charge is ether screw-fed to the rotating device or dropped directly into the stationary device. Both versions of the device were used with equal success in this work. The heated powder is discharged via a pneumatically activated hatch in the bottom of the horizontal chamber. The horizontal chamber is double-walled such that cooling or heating fluid can maintain the chamber wall at a predetermined temperature. The chamber is operated at atmospheric pressure and any reactive by-products or dissolved vapors are vented through the end of the chamber. Tip speed is a function of the size of the heating chamber and the shaft speed. Speed control was achieved by manually changing the motor revolutions/minute, based on a preset time sequence. More accurate and reliable relay- and computer-controlled sequencing is an envisioned aspect of this invention.

The exact form of the mechanical heating device is not restricted to the commercially available Draiswerke Gelimat. Other mechanical heating devices having timed control of shaft speed and predetermined chamber wall temperature control are envisioned aspects of this invention.

As an example of the novel way of heating polymers in a mechanical heating device, Torlon.TM. was used as an example of a reactive polyimide. Torlon.TM. is available in a partially imidized state as Torlon 4000T.TM. powder. The powder is characterized as light yellow and characteristically is injection moldable in that state. In order to achieve optimum final property conditions, the molded part is then carefully heated in a ramping temperature oven over several hours to days. This heating profile allows complete solid state imidization to occur in the polymer. Fully imidized Torlon has a dark brown to near-black color. During imidization, water vapor, a by-product of the reaction, is diffused from the molded part. A measure of the completeness of imidization is the glass transition temperature of the polymer. The glass transition temperature of as-received Torlon 4000T .TM. is approximately 230.degree. C.. The glass transition temperature of fully imidized Torlon is approximately 275.degree. C.

In a set of tests using Torlon polyamide-imide, the Gelimat G1S was charged with 200 g samples. The Gelimat chamber wall temperature was measured to be 175.degree. C..+-.10.degree. C. The motor speed was maintained at 7000 revolutions/minute. The charges were dropped at fixed times and the charge temperature measured with a handheld thermocouple having a needle-nose probe. The measured temperatures are shown in Table I. As is apparent, the heating rate is linear with time, at 2.degree. C./second.

In another series of tests, batches of Torlon polyamide-imide ranging in weights from 50 g to 250 g in 50 g increments were processed in a Gelimat G1S mechanical heating device. The chamber wall temperature was measured at 200.degree. C..+-.5.degree. C. for these tests. The batches were charged directly into the device with the motor speed at 7000 revolutions/minute. The charges were manually discharged in 9.0.+-.1.0 seconds and the batch temperatures were measured with a handle thermocouple having a needle-nose probe.

The results in Table II show that the heating rate does not depend on batch size over the range of 50 to 250 g per liter volume of the chamber. For this chamber temperature, the heating rate is approximately 30.degree. C./second.

In yet another series of tests, batches of Torlon polyamide-imide were processed in a Gelimat G1 mechanical heating device. For this device, a Vanzetti infrared fiber optic device was used to monitor internal temperature. The device had a capability of sensing a maximum temperature of 410.degree. C. From a series of experiments, the measured temperature was determined to be approximately 50.degree. C. to 60.degree. C. above actual polymer temperature, since the actual powder temperature was 350.degree. to 360.degree. C. at an indicated temperature of 410.degree. C. It was determined that the heating rate of 200 g charge between 350.degree. and 400.degree. C. was 10.degree. C./second. As a result, every charge was discharged from the unit at an indicated temperature of 400.degree. C. plus a nominal 4 seconds depending on the particular testing conditions. Powder temperature was determined to be 390.degree. C..+-.5.degree. C. at that condition. As shown in Table III, the novel processing procedure was used to mold 4-inch diameter disks.

During the ramp up to the hold temperature, the Gelimat G1 frequency control was set at approximately 50% of the maximum setting or approximately 30 Hz. At that setting according to the 1991 Draiswerke Operating Manual, the motor speed was approximately 3200 revolutions/minute. During the hold period of up to 60 seconds, the frequency control was set at approximately 40% of the maximum setting or approximately 25 Hz. At that setting according to the 1991 Draiswerke Operating Manual, the motor speed was approximately 2600 revolutions/minute. These settings were manually achieved throughout the experiments described herein. With proper closed loop control, the preferred settings for the preferred periods of time could be achieved electronically. As seen in Table III, the maximum speed achievable by the mechanical heating device, to wit, 6400 revolutions/minute, was not required to fully heat the powder to the desired molding temperature of approximately 380.degree. C.

Throughout the ramping and holding sequence, steam was observed exiting the vent pipe mounted in the face plate of the mechanical heating device. This steam is the moisture vapor liberated by polyamide-imide during its imidization reaction. The observed rate of venting of steam was used to obtain a desired time-temperature ramping sequence of approximately 5.degree. C./second.+-.3.degree. C./second. The polyamide-imide powder was then held at the holding temperature of 360.degree. C..+-.10.degree. C. as indicated by the Vanzetti device, most probably at 300.degree. C..+-.10.degree. C. actual powder temperature, for a period of time sufficient to observe a near-cessation in the rate of steam venting. The near-cessation of steam venting was taken to be a direct measure to the near-completion of the imidization reaction. This time varied from approximately 10 seconds for charge weights of 100 g or less to 30 seconds for charge weights of 200 g or so. It was determined that regardless of the charge weights below about 250 g per liter volume of the mechanical heating device, hold times of 30 seconds were sufficient to achieve near-cessation of steam venting.

The discharged heated powder was then transferred manually to a 80T compression molding press. Several molds were used in this study, including a steam-heated 2-inch diameter disk mold, a steam- and electrically-heated 4-inch diameter disk mold, and an electrically-heated 5-inch diameter ashtray mold. The maximum mold temperature achieved for each of the molds is shown in Table IV. It is apparent that a mold temperature range of 425.degree. to 500.degree. F. is preferred with a most preferred range of 450.degree. to 475.degree. F. It is understood that this compression molding temperature range must be determined for each polyimide mechanically heated and molded in the manner described in this invention.

Several tests were conducted in order to determine whether the polyamide-imide was reacted using this novel approach. First, a 2-inch disk molded as #3/218 using a heating protocol similar to #13/218, was cut in half with a band-saw. Half the sample was placed in a 500.degree. F. [260.degree. C.] dry air oven for 48 hours. The sample was then cooled and its thickness and diameter measured and compared with the unheated half. No difference to .+-.0.005 inches was observed. It is know that as-molded Torlon 4000T cannot withstand 500.degree. F. air environment without experiencing substantial distortion. Then, the density of the unheated half of the sample was obtained. The measured density of 1.37 g/cm.sup.3 compares well with literature values of 1.40 g/cm.sup.3. Then, small pieces of the unheated and oven-heated samples were subjected to differential scanning calorimetry to determine transition temperatures. The result are shown in Table V. The glass transition temperature of sample #3/218 is about the same as that for literature value of the glass transition temperature of fully cured Torlon.TM. polyamide-imide. The glass transition temperature of that same sample is increased slightly by additional heating for 48 hours in a 260.degree. C. oven. It is apparent that the technique used in the instant invention allows for simultaneously heating and reacting polyamide-imide powders to molding conditions, thereby circumventing the current cumbersome and unreliable technique of molding the partially imidized polyamide-imide, then solid-state imidizing the polymer in the final molded form.

In yet another set of experiments, Lenzing compression molding polyimide powder designated as P84-HCM.TM. [40/100 mesh] was heated to a Vanzetti set point temperature of 400.degree. C.+4 seconds, then compression molded into 4-inch diameter disks using an isothermal steam-heated mold at 240.degree. F. [115.degree. C.]. In the first trials, the powder was heated directly to molding condition without delay. The powder could not be compacted into a useful disk. When the powder was heated in the fashion described above for the polyamide-imide, the polyimide could be molded into solid disks, albeit with granular surfaces owing to the low mold temperature. The results are shown in Table VI.

A representative heating rate for P84-HCM polyimide powder is shown in Table VII. The heating rate profile is similar to that used to heat the reactive polyamide-imide. During the heating and holding sequences, the polymer powder off-gassed in much the same way as the reactive polyamide-imide powder. Although the exhausted vapor appeared to be steam and was condensed on a cold metallic surface, the condensate was not analyzed. This off-gassing was unexpected. However, it appeared to be necessary since attempts to mold polyimide powder that had not been heated in the proscribed programmed fashion were unsuccessful. The density of a section cut from #10/224 was determined to be 1.33.+-.0.01 g/cm.sup.3. The literature value for P84 polyimide is 1.34 g/cm.sup.3.

The 4-inch diameter disks designated as Run #5/224 and Run #6/224 were sent to Lenzing for material property evaluation. Table VIII summarizes the Lenzing test report.

In Tables III, VI and VII, the Vanzetti infrared temperature sensor indicated temperatures that were approximately 50.degree. C. to 60.degree. C. above actual temperature. In all tests described here, pressure on the heated powder was applied using an 80T compression molding press and pressures of approximately 1,000 to 2,000 lb.sub.f /in.sup.2.

Whereas compression molding of disks and shapes is the only secondary processing step described herein for the forming of shapes of polyimides beginning with powder, it is obvious to anyone skilled in the art that other types of shaping and forming are also possible using the powder heated in the manner described in this patent. As an example, the hot powder can be charged directly to a transfer mold, wherein a heated chamber fitted with a high speed piston transfers the heated powder to cavities in a compression mold. And in another example, the hot powder can be charged directly to the inlet of a ram extruder for the production of continuous shaped articles. And in yet another example, the hot powder can be consolidated into a monolithic sheet by introducing the powder directly in the nip between two high pressure rolls.

Although the above describes the preferred embodiments of the invention, many variations and amendments to the process described herein can be made without departing from the scope of the invention.