Flame-Resistant Resins Fit Harsh Applications

High-temperature composite materials that are easily processed and exhibit high thermal and oxidative stability are in increasing demand for advanced aerospace and marine applications. The search for polymeric materials that can bridge the gap between currently used high-temperature polymers and metals/ceramics is of high importance due to the advantages inherent to organic polymers: low density, processing ease, and enhanced composite mechanical properties when considering strength-to-weight ratios.

The key to synthesizing high-temperature organic-based polymers is to use highly stable molecular units such as aromatic and/or heterocyclic rings within the backbone. Unfortunately, polymeric materials composed entirely of these units tend to be inherently insoluble and intractable. Thus, flexible linkages must be introduced into the polymeric system, sacrificing the thermal stability to enhance the processability. The real problem has been to devise synthetic methods for linking these groups through flexible units. These units must be versatile and remain independent from side reactions while retaining excellent thermal and oxidative properties.

Phthalonitrile-Based Polymers

To meet the increasing need for advanced composites for high-temperature applications, phthalonitrile-based polymers with exceptional thermal, oxidative and mechanical properties are under development at the Naval Research Laboratory (Washington, DC). These resins are showing superior properties over current state-of-the-art, high-temperature thermosetting polymers.

The composites are easy to process from indefinitely stable prepreg, exhibit high thermal and oxidative stability approaching 375°C (700°F) in air, have low-equilibrium moisture pickup, and show fire tolerance that exceeds Navy specifications for composite-ship applications. Additionally, the cure exotherm is easily controlled for thick composite fabrication. Phthalonitrile/CCA-3 composites lose less weight than baseline carbon-cloth phenolic materials when exposed to plasma-torch testing.

Phthalonitrile resins can be used in hardy, advanced composites such as those incorporated into engines, missiles, and rockets. The resins can also be used in high-temperature adhesives, dielectric materials for electronic applications, and intrinsic electrically conducting polymers.

Synthesis And Polymerization

Phthalonitrile polymers (above) prepared from a polymerization of 4,4'-bis (3,4dicyanophenoxy)biphenyl 1A and 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane 1B, show excellent thermal and oxidative stability at temperatures approaching 372°C (700°F). The phthalonitrile monomers are readily converted to highly crosslinked thermosetting polymers in the presence of thermally stable organic amines, phenols, or strong acids. Polymerization occurs by an addition reaction through the terminal phthalonitrile units to afford void-free polymetric materials. The properties of the polymer are controlled as a function of the amount of curing additive used and the curing temperature (>200°C).

Processing

The thermosetting polymers are easily processed into shaped components or films in a controlled manner. After the addition of the curing additive, the reaction mixture is rapidly converted from a crystalline into an amorphous phase. At this stage, the curing temperature can be lowered to a point above the glass-transition temperature (Tg) of the prepolymer. The polymerization reaction can be performed in one step by heating the melt of the curing mixture above Tg until gelation occurs.

When postcured at 375°C, the resulting polymers do not show any evidence of glass/rubber transition upon analysis to 375°C. Alternatively, low molecular weight prepolymers, which can be stored indefinitely under ambient conditions, can be formed by quenching the reaction before gelation occurs. The amorphous prepolymers are soluble in common solvents such as methylene chloride, chloroform, and the dipolar aprotic solvents. Due to the solubility properties and indefinite stability at ambient temperatures, the prepolymers formed from these monomers are potential candidates for the preparation of stable prepregs and their application as laminates for advanced fiber-reinforced composites.

Thermal Stability

The thermal and oxidative stabilities have been assessed under isothermal and dynamic conditions. Isothermal measurements in an air flow at 100 cc/min indicate that the polymers can be expected to perform well for relatively long exposures at moderate temperatures between 300-357°C (572-675°F). The thermal and oxidative properties were found to be enhanced as the polymers were postcured at elevated temperatures. Catastrophic failure in air consistently occurred between 525-600°C (980-1110°F).

Mechanical Properties

The phthalonitrile-based polymers have been shown to exhibit excellent mechanical properties. Compared with other state-of-the-art thermosetting polymers, the phthalonitrile polymers display superior tensile strength values and retain these values when aged at 315°C (600°F) for extended periods in an oxidizing environment (see table below). The phthalonitrile polymers have also been found to exhibit a fracture toughness similar to the values of unmodified epoxy resins.

Electrical Conductivity

The excellent thermal stability displayed by the phthalonitrile polymers contributed to further pyrolytic studies, These studies were performed on the resin prepared from polymerizing 4,4'-bis (3,4-dicyanophenoxy)biphenyl 1A. The electrical behavior can be systematically changed from an insulator to a semiconductor and made to approach metallic regions by controlling the thermal-processing temperature. For pryolytic temperatures up to 500°C, the conductive nature changes from an insulator and approaches the semiconductive region. At 900°C, the conductivity approaches the metallic regions. Although there was some shrinkage and a weight loss of 28% at 900°C, the pyrolytic material retained its structural integrity and formed minimal microvoids.

Summary

The phthalonitrile polymers are potential high-temperature materials for structural composite applications. The syntheses are short and potentially low-cost. The resin is easily processed into void-free components. The polymers exhibit high thermal and oxidative stability, low flammability, high char yields, and good mechanical properties, and can be made to exhibit electrical conductivity at elevated temperatures. Prepolymer formation is readily achievable in a controlled manner. The payoff could be especially high in space and military applications where the low weight and corrosion resistance of plastics are important. The phthalonitrile technology has been licensed by Cardolite Corp. (Newark, NJ).

For more information, call 202-767-3095.