Donald J. Progar

Preliminary Evaluation of Hybrid Titanium Composite Laminates

In this study, the mechanical response of hybrid titanium composite laminates (HTCL) was evaluated at room and elevated temperatures. Also, the use of an elastic-plastic laminate analysis program for predicting the tensile response from constituent properties was verified. The improvement in mechanical properties achieved by the laminates was assessed by comparing the results of static strength and constant amplitude fatigue tests with those for monolithic titanium sheet. Two HTCL were fabricated with different fiber volume fractions, resin layer thicknesses and resins. One panel was thicker and was poorly bonded in comparison with the other. Consequently, the former had a lower tensile strength, while fewer cracks grew in this panel and at a slower rate. Both panels showed an improvement in fatigue life of almost two orders of magnitude. The model predictions were also in good agreement with the experimental results for both HTCL panels.

A new flexible backbone polyimide adhesive

A new linear, aromatic, thermoplastic polyimide, identified as LARC-IA, has been synthesized and evaluated, primarily as an adhesive, which is physiologically safe and relatively inexpensive. The polymer was prepared from oxydiphthalic anhydride (ODPA) and 3,4-oxydianiline (ODA) in diglyme. In order to obtain optimal flow properties, which improves wetting, the molecular weight of the polymer was controlled by use of a monofunctional anhydride, phthalic anhydride (PA). Adhesively bonded lap shear specimens, using Ti-6AI-4V adherends, were prepared and tested to assess its adhesive potential. Specimens were exposed to water boil and thermal aging to determine the adhesive systems durability. Flatwise tensile strength and critical fracture energy were also determined. Results were compared to data for LARC-TPI. Preliminary flexural strength and modulus results were also obtained for composites fabricated from LARC-IA (5% PA) and graphite fibers (AS4, 12K). Initial results of this study indicate considerabl...

Evaluation of a novel thermoplastic polyimide for bonding titanium

Abstract A novel hot-melt processable polyimide has been synthesized and characterized as an adhesive for a titanium alloy. This system shows potential for applications with service temperatures near 200°C. A bonding cycle was developed from preliminary processing studies. The bonded specimens were tested at room temperature, 177°C and 204°C both before and after aging in air at 204°C for times up to 1000 hours. There was no appreciable change in strength after aging. Bonded specimens were exposed to boiling water continously for a 72 hour period. These specimens exhibited some loss in strength due to this rather severe exposure.

Flexible Backbone Aromatic Polyimide Adhesives

Abstract Continuing research at Langley Research Center on the synthesis and development of new inexpensive flexible aromatic polyimides as adhesives has resulted in a material identified as LARC-F-SO2 with similarities to polyimidesulfone (PISO2) and other flexible backbone polyimides recently reported by Progar and St. Clair. Also prepared and evaluated was an endcapped version of PISO2. These two polymers were compared with LARC-TPI and LARC-STPI, polyimides researched in our laboratory and reported in the literature. The adhesive evaluation, primarily based on lap shear strength (LSS) tests, involved preparing adhesive tapes, conducting bonding studies and exposing lap shear specimens to 204°C air for up to 1000 hrs and to a 72-hour water boil. LSS tests at RT, 177°C and 204°C were performed before (controls) and after these exposures. The type of adhesive failure as well as the Tg was determined for the fractured specimens. The results indicate that LARC-TPI provides the highest LSSs, 33 MPa at RT, 3...

Adhesive evaluation of water-soluble LARC-TPI

Abstract A water-soluble version of the linear thermoplastic polyimide LARC-TPI, prepared by United Technologies Research Center, East Hartford, CT, USA from the polyamic-acid/ diglyme solution commercially marketed by Mitsui Toatsu Chemicals Incorporated (MTCI), Tokyo, Japan, was evaluated as a high temperature thermoplastic adhesive. A water-solvent polyimide system has the advantages of low solvent cost and environmental acceptibility due to non-toxicity during manufacture and processing. This report details a study to evaluate the water-soluble polyimide, identified as TPI(MTC)/H2O), as a high temperature thermoplastic adhesive for bonding Ti6Al4V. The results are compared primarily with those reported in earlier work with the polyamic-acid/diglyme material supplied by MTCI. In general, it is shown that the TPI(MTC)/H2O retains high lap shear strengths after thermal exposure but has reduced strengths after the water-boil exposure. All failures were cohesive. The TPI(MTC)/H2O compared very well with previous data reported for the standard polyamic-acid/diglyme LARC-TPI and, therefore, shows promise as a water-soluble adhesive for use in various applications.

Flexibilized Copolyimide Adhesives

Abstract Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI. Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours. The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)]. These flexible th...

Adhesive bonding study of amorphous LARC-TPI

A novel form of LARC™-TPI has been developed which does not exhibit a crystalline melt as was encountered in earlier fonns of this material. Being amorphous allows the powder to be consolidated at temperatures near its glass transition temperature of 260°C (500°F). Adhesively bonded titanium lap shear specimens were fabricated at temperatures as low as 260°C (500°F). At lower temperatures, the pressures needed to create a well-consolidated bond were high, but as the bonding temperature was increased, the pressure could be decreased. High quality, high strength bonds were prepared at a temperature as low as 280°C (536 °F) with a pressure of only 0.69 MPa (100 psi). At a 350°C (662°F) bonding temperature, only 0.10 MPa (15 psi) was needed to develop very high lap shear strengths.

Adhesive evaluation of LARC-TPI and a water-soluble version of LARC-TPI

Abstract A linear thermoplastic polyimide, LARC-TPI, developed several years ago at NASA Langley Research Center, showed promise as a high temperature adhesive for application in aircraft and spacecraft based on the materials toughness, flexibility, good thermal and thermo-oxidative stability, and other attractive properties. Since then, a commercial product manufactured and supplied by Mitsui Toatsu Chemicals, Inc, Tokyo, Japan, has become available. In addition, a water-soluble version containing N,N-dimethylethanol amine salt was supplied by United Technologies Research Center, East Hartford, CT, USA. The water-soluble version is highly attractive because of the safety requirements involved with organic solvents presently used for polyimide materials. This report details the results of a study to evaluate the two LARC-TPI materials as high temperature thermoplastic adhesives and primers for bonding titanium (6Al-4V) adherends. A limited characterization of the materials was performed using a diffuse reflectance-Fourier transform infrared spectroscopy technique. Thermomechanical analysis and torsional braid techniques were used to determine the glass transition temperature. The effects of long-term thermal exposure (up to 1000 h) at 204°C and 72 h water-boil on the strength of the adhesives was determined by simple lap shear tests.

Processing study of a high temperature adhesive

An adhesive-bonding process cycle study was performed for a polyimidesulphone. The high molecular weight, linear aromatic system possesses properties which make it attractive as a processable, low-cost material for elevated temperature applications. The results of a study to better understand the parameters that affect the adhesive properties of the polymer for titanium alloy adherends are presented. These include the tape preparation, the use of a primer and press and simulated autoclave processing conditions. The polymer was characterized using Fourier transform infrared spectroscopy, glass transition temperature determination, flow measurements, and weight loss measurements. The lap shear strength of the adhesive was used to evaluate the effects of the bonding process variations.

Solvent and Structure Studies of Novel Polyimide Adhesives

Significant improvements in condensation polyimide adhesives have been made by chemical modification of the diamine or dianhydride monomers, by use of copolymer composition and by use of a unique ether solvent as the polymerization medium. The effect on adhesive properties was studied and a family of adhesives developed whose bonding pressures (40-200 psi) and use temperature (up to 300°C) can be varied with composition to cover a wide range of practical applications. Alterations in the polymer backbone have also led to imidized films that have sufficient flow for possible use as film adhesives.