Erik S. Weiser

Polyimide Foams for Aerospace Vehicles

Due to a demand by the aerospace industry, NASA has begun developing the next generation of polyimide foams which could be utilized to reduce vehicle weight for the X-33 and Reusable Launch Vehicle (RLV) programmes. The activity at NASA Langley Research Center focuses on developing polyimide foam and foam structures which are made using monomeric solutions or salt solutions formed from the reaction of a dianhydride and diamine dissolved in a mixture of foaming agents and alkyl alcohols. This process can produce polyimide foams with varying properties from a large number of monomers and monomer blends. The specific densities of these foams can range from 0.008 g cc−1 to 0.32 g cc−1. Polyimide foams at densities of 0.032 g cc−1 and 0.08 g cc−1 were tested for a wide range of physical properties. The foams demonstrated excellent thermal stability at 321°C, a good thermal conductivity at 25°C of 0.03 W m−1 K−1, compressive strengths as high as 0.84 MPa at 10% deflection and a limiting oxygen index of 51%. Thermomechanical cyclic testing was also performed on these materials for 50 cycles at temperatures from −253°C to 204°C. The foams survived the cyclic testing without debonding or cracking. Thermal forming of the 0.032 g cc−1 foam was performed and a minimum radius curvature of 0.0711 m was achieved. The foams exhibited excellent properties overall and are shown to be viable for use as cryogenic insulation on the next generation RLV.

Characterization of polyimide foams after exposure to extreme weathering conditions

The weathering degradation of three closely related polyimide foams was studied by X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared (FT-IR) spectroscopy, Raman spectroscopy, Thermogravimetric Analysis (TGA) and Thermomechanical Analysis (TMA) after exposure at the NASA Kennedy Space Centers (KSC) Beach Corrosion Site. These foams were developed by NASA Langley Research Center for applications such as cryogenic insulation, flame retardant panels and structural subcomponents. The degradative environmental conditions at the KSC corrosion site include exposure to sunlight, exposure to changes in temperature and humidity, mechanical erosion by wind and rain, and high sodium chloride content due to the close proximity of the ocean. Other possible atmospheric contaminants include hydrogen sulfide and hydrogen chloride (the latter originating with exhausts from the launching of space vehicles). The foams were studied for a total of 17 months exposure, with samples taken at 3, 9 and 17 months. Data analyses of the weathered foams showed that chemical structure and density effects were the key variables in weathering performance. The carbonyl linkage in the dianhydride of the TEEK-L series polyimide foams is the most important factor in degradation. TEEK-H series foams, which contain an ether linkage in the dianhydride, showed much less degradation or more resistance to weathering in comparison to the TEEK-L series. In the same chemical series, the lower density foams were more degraded in comparison to higher density foams.

Processing Characteristics of TEEK Polyimide Foam

High performance polymeric foams have experienced a growing demand in aerospace applications such as cryogenic insulation, flame-retardant panels and structural sub-components. The Next Generation Launch Technology (NGLT) program requires foams capable of retaining their structural integrity while providing insulating capabilities in a liquid hydrogen environment. NASA Langley Research Center (LaRC) and Unitika Ltd co-developed a polyimide foam processed from a fine powder of salt-like foaming precursor isolated from solution via solvent evaporation. This paper reports the foaming characteristics of this precursor based on rheometric, calorimetric, thermogravimetric and chromatographic characterization methods. The degree of foaming varied significantly as a function of process heating rate and hold temperatures. Results from the battery of measuring techniques employed afford a complete picture of the foaming and curing mechanism for the subject polyimide foam.

Polyimide Composites from ‘Salt-Like’ Solution Precursors

Four NASA Langley-developed polyimide matrix resins, LaRC™-IA, LaRC™-IAX, LaRC™-8515 and LaRC™-PETI-5, were produced via a ‘salt-like’ process developed by Unitika Ltd. The salt-like solutions (65% solids in NMP) were prepregged onto Hexcel IM7 carbon fibre using the NASA LaRC™ multipurpose tape machine. Process parameters were determined and composite panels fabricated. The temperature dependent volatile depletion rates, the thermal crystallization behaviour and the resin rheology were characterized. Composite moulding cycles were developed which consistently yielded well consolidated, void-free laminated parts. Composite mechanical properties such as the short beam shear strength; the longitudinal and transverse flexural strength and flexural modulus; the longitudinal compression strength and modulus; and the open hole compression strength and compression after impact strength were measured at room temperature and elevated temperatures. The processing characteristics and the composite mechanical properties of the four intermediate modulus carbon fibre/polyimide matrix composites were compared to existing data on the same polyimide resin systems and IM7 carbon fibre manufactured via poly(amide acid) solutions (30–35% solids in NMP). This work studies the effects of varying the synthetic route on the processing and mechanical properties of the polyimide composites.

Solid-state polyimide foaming from powder precursors: Effect of morphology and process parameters on the diffusive phenomena

Solid poly(amic acid) (PAA) precursor is used for foaming into microspheres or foams depending on whether the powder particles are free or confined inside a mold. Solid-state foaming of powder precursors is studied by examining concurrent and competitive phenomena that determine the morphology and physical properties of the foam unit cell. Simultaneous analysis through thermo gravimetric analysis (TGA) and modulated differential scanning calorimetry (MDSC) is proposed for the characterization of important transitions during the blowing process. Effects of particle size and shape on bubble growth will be addressed.

Validation of a polyimide foam model for use in transmission loss applications

In this paper, the use of polyimide foam as a lining in double panel applications is considered. Polyimide foam has a number of attractive functional attributes, not the least of which is its high fire resistance, thus making its use desirable in some sound transmission applications. The configuration studied here consisted of two 0.04×94 thick, flat aluminum panels separated by 5 in., with a 3 in. thick layer of foam centered in that space. Random incidence transmission loss measurements were conducted on this buildup, and conventional poro‐elastic models were used to predict the performance of the lining material. The Biot parameters of the foam were determined by a combination of direct measurement (for density, flow resistivity and Young’s modulus) and inverse characterization procedures (for porosity, tortuosity, viscous and thermal characteristic length, Poisson’s ratio, and loss factor). The inverse characterization procedure involved matching normal incidence standing wave tube measurements of absorption coefficient and transmission loss of the isolated foam with finite element predictions. When the foam parameters determined in this way were used to predict the performance of the complete double panel system, reasonable agreement between the measured transmission loss and predictions made using commercial statistical energy analysis codes was obtained.