Thermoplastic polyimide processing

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

Poly[2,2 -(m-phenylene-5,5 -benzimidazole)] (PBI) is a very high glass transition temperature (Tg 430°C), commercially available material. It possesses excellent mechanical properties, but is difficult to process into large parts and has high moisture regain and poor thermo-oxidative stability at temperatures above approximately 260 °C. Polyimides, especially the thermoplastic polyimides, offer attractive thermo-oxidative stability and processibility, but often lack the thermal and mechanical characteristics necessary to perform in applications such as the matrix for high use-temperature (over 300 °C) structural composites (for example, carbon fiber reinforced) for aerospace use. The attempt to mitigate... 

Certain thermoplastic polyimides possess excellent resistances to high temperatures and chemicals, with Tgs ranging from 217 to 371 °C. Certain polyimides also exhibit excellent toughness and dielectric properties. The melt blending process of polyimides with other thermoplastic polymers is difficult due to polyimides high Tg, high melt viscosities, and incompatibility. A solution process is used, therefore, to achieve a semi-interpenetrating polyimide network... 

Not only do the chemical structure and the molecular weight affect the processability but also the method of synthesis, in particular the imidation step. Thermally imidized polyimides are always less tractable than solution imidized polyimides. That is because thermally imidized polyimides can undergo cross-linking, and because thermal treatment (about 300 °C) favour chains packing and provide higher molecular order than that achievable by solution imidation. Therefore, solution imidation is always preferable when thermoplastic polyimides are to be developed. 

In general, these materials produced insoluble systems after imidization and had to be processed by first casting the amic acid intermediate onto a substrate, then imidizing. More recently [8], many examples have been demonstrated where the resulting polyimide is thermoplastic, melt processable and thus can ... 

NASA-Langley provided a further improvement in processibility by replacing the methylenedianiline with a commercial aromatic diamine mixture that afforded even better handling characteristics (17). This type of product is formed by the reaction of formaldehyde with aniline as a precursor in isocyanate manufacture. Fluorine-containing thermoplastic polyimides and polyimides end capped with acetylene groups for cross-linking are also available as potential commercial high-temperature resins. 

The cured or fully imidized polyimide, unlike the poly(amic acid), is insoluble and infusible with high thermooxidative stability and good electrical-insulation properties. Thermoplastic polyimides that can be melt processed at high temperatures or cast in solution are now also available. Through an appropriate choice of the aromatic diamine, phenyl or alkyl pendant groups or main-chain aromatic polyether linkages can be introduced into the polymer. The resulting polyimides are soluble in relatively nonpolar solvents. 

Several major advances in polyimide chemistry have reduced some of the process difficulties. By end capping the polyimide precursor with acetylenic unsaturation, by-product free addition cures can be achieved. The development of a thermoplastic polyimide eliminates the high-temperature cure requirement [50]. Both of these... 

Thermoplastic polyimides are available with similar high-temperature and high-performance properties as the thermosets but can be formed using typical melt-processing techniques such as injection moulding. The materials can be used as unfilled base polymer, glass or carbon fibre... 

In 1965, workers from DuPont first prepared a linear thermoplastic polyimide. When aromatic diamines are reacted with aromatic dianhydrides, aromatic tetracarboxylic acids, or dialkyl esters of aromatic tetracarboxylic acids, in a solvent such as DMF (Table 13.7), they were converted to a polyamic acid, which when heated, cured by cyclization (imide ring closure, or imidization). This ring closure is a condensation process, releasing either water, or an alkyl alcohol, or both [36]. 

Introducing a thermoplastic resin [53,54] into the matrix such as a thermoplastic polyimide Matrimid 5218 [55,56], polyethersidfone [54-57], polyetherimide [55-56,58-61] and poly (arylene ether) [58-59,61-63]. All these polymers however, cause an attendant loss of processability coupled with poorer solvent resistance. 

Thermoplastic Polyimides. Several types of linear high-molecular-weight polyimides have been developed, which contain enough single bonds in the polymer backbone to make them somewhat flexible and therefore usable in conventional thermoplastic melt processing (Fig. 3.40). This does, of course, sacrifice some of the inherent thermal stability of polyimides (Table 3.45). 

A partial summary of commercial polyetherimide grade names and their suppliers is listed in Table 8.1. Table 8.2 shows properties of some of the commercial, fully amorphous, melt-processable thermoplastic polyimides listed in Table 8.1. They have high heat capabflity and excellent strength [23]. 

In fully imidized thermoplastic polyimides, the material is fully imidized before final processing. By its very nature, a fully imidized thermoplastic, i.e., CIBA-GEIGY s XU 218 and Upjohn s Polyimide 2080, remain soluble and fusible. As such, fully imidized thermoplastic polyimides can be molded into final parts without undergoing any further chemical reaction. Some thermal stability is, however, sacrified. 

Compression molding involves placing a material, usually in powder form, into the cavity of a heated mold (250OC) and applying heat (up to 350 C) and pressure (3000-5000 psi).56 This processing method can be used for the addition-type and thermoplastic polyimides. 

Further improvements in the chemistry of polyimides during the last years have been directed towards novel, linear species that are soluble in workable organic solvents or melt-processable while fully imidized. Thus, changes had to be introduced in the chemical structure to adapt the behaviour and performance of these specialty polymers to the demands of the new technologies. As a consequence, a new generation of condensation polyimides has appeared, the so-called thermoplastic polyimides. 

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