‘Self-Reinforcing’ Thermoplastic Is Harder, Stronger, Stiffer Without Added Fibers

By the end of 2003, sample quantities will become available of a specialty high-performance thermoplastic whose inventors call it the first readily processable rigid-rod polymer. Named Parmax SRP (“self-reinforcing polymer”), it lays claim to a striking list of superlatives: Compared with any other thermoplastic, it is said to be the hardest, strongest, stiffest, most inherently flame retardant, highest in refractive index, and lowest in thermal expansion coefficient (CTE). It is also the most solvent resistant, though PPS comes close. These properties suggest market potential in the most demanding applications such as military equipment, jet engines, electronics, chemical processing, oil-field components, and structural foam cores for sandwich composites. Longer-term prospects include optical parts and glazing.

Parmax SRP is the sole product of Mississippi Polymer Technologies (MPT), an R&D company that is gradually scaling up to commercial production. Founded in 2000, it currently has a staff of 27, including 18 engineers, chemists, and polymer scientists. The staff has doubled in the past year and is expected to grow to 40 by the end of 2003. At least one of the senior staff, Dr. John Flock, is an alumnus of GE Plastics.

MPT currently produces Parmax SRP in a pilot plant capable of 100,000 lb/yr. The company hopes to scale up to commercial capacity of 1 million lb/yr by 2004 and perhaps many times that amount in 2005. At the million-pound production level, pricing is expected to be $30 to $35/lb, and long-term pricing would be $6 to $8/lb.

Parmax SRP development has been supported largely by a series of contracts from the Defense Dept., and much of MPT’s initial commercial capacity will be dedicated to military projects such as missile parts, launch tubes for shoulder-fired weapons, jet engines, and structural composites for naval vessels.

 

New kind of polymer

Parmax SRP is a highly aromatic resin derived from plentiful, inexpensive chlorobenzene. It is based on a string of substituted and unsubstituted phenylene rings that produce a highly rigid structure. This gives Parmax SRP superior structural properties without the need for added reinforcing fibers. In fact, small amounts of this resin can be used to reinforce other polymers.

Parmax SRPs are very different from liquid-crystal polymers (LCPs). Parmax SRPs are completely amorphous, not crystalline. They are transparent, where LCPs generally are opaque. Also, Parmax SRPs are isotropic and produce homogeneous molded parts, whereas LCPs are highly anisotropic and produce parts with a fibrillar structure.

Although Parmax SRP is not the first rigid-rod polymer to have been developed (others are PBI and PBO), it is the first that lends itself readily to solvent and melt processing into films and shapes, according to Nick Malkovich, v.p. of product development. This resin is soluble in many common solvents, allowing it to be cast into films and coatings. It can also be made into pellets and powders for compression molding and extrusion. Extruded tubes have “phenomenal burst strength,” notes Dr. Robert Springfield, v.p. of production. Compression molding requires pressing at 572 to 662 F and at least 100 psi. Parmax SRP has also been foamed with CO2 and nitrogen.

Current developmental grades have very high viscosity—twice that of PEEK, for example—making injection molding a challenge. An injection molding grade is in development and expected to be available by mid-2004.

Extruded stock shapes can be ma chined with standard equipment and bits used for plastics and laminates. Parmax SRP is said to behave generally like aluminum in machining operations and is able to hold tolerances of less than 1 mil. It won’t chip or melt and it tolerates cutting oils.

Blends and alloys are expected to be a major outlet for Parmax SRP. It is highly miscible with polycarbonate and polysulfone. Clear films have been cast from solvent blends with these resins and with certain polyimides.

Parmax SRP is said to show remarkable heat stability without any stabilizers or additives. It is stable at 400 C (750 F) and has been extruded and reprocessed 10 times with no apparent loss of molecular weight or properties.

 

Striking property profile

Although some 1100 varieties have been produced, commercial development is currently focusing on two basic grades, Parmax 1000 and 1200. As shown in the accompanying property table, these resins are two to four times stiffer and two to three times stronger than any other thermoplastic. Their surface hardness is also greater, providing excellent scratch and wear resistance. Together with very low coefficient of friction and compressive strength that can be greater than 100,000 psi, these qualities suggest possible uses in polymer ball bearings.

Although current Parmax SRP grades have glass-transition temperatures up to 165 C (330 F), versions have been produced with Tg up to 270 C (518 F). At the same time, the resin does not lose its strength at cryogenic temperatures—e.g., when submerged in liquid nitrogen.

Despite its very high stiffness, Parmax SRP boasts higher impact strength than PEEK or polyetherimide (GE’s Ultem). Recent developments have raised unnotched Izod impact strength as high as 25-30 ft-lb/in. Also, Parmax SRP reportedly is far more ductile than typical fiber-reinforced thermoplastics.

Although transparent, Parmax SRP shows the yellow tint typical of aromatic polymers. Its color is said to be lighter than that of GE’s Ultem, and Malkovich thinks it can ultimately be made lighter than standard polysulfone. A 50/50 blend with polycarbonate shows only a faint color that could be masked with a blue die, according to Malkovich. He adds that the extremely high refractive index of Parmax SRP could eventually permit molding of much thinner eyeglass lenses.

Inherent flame resistance of Parmax SRP was demonstrated by holding it up to an acetylene torch for 15 minutes. “It never ignited, never smoked, just charred,” reports Malkovich.