Unreinforced Plastics Get Stiffer

Engineers interested in replacing metals with plastics now have a new weapon in their arsenal. Called Primospire, it’s the stiffest unreinforced thermoplastic currently available commercially.

The strongest, stiffest Primospire grade has a flexural modulus of 1.2 million psi and a tensile strength of 30,000 psi. These values make it about two to three times stiffer and stronger than the most commonly used engineering plastics (see chart). And this off-the-chart mechanical performance comes from a “neat” or unmodified resin. “Even neat, Primospire is actually stronger than many glass-reinforced plastics,” notes Jamal El-Hibri, a senior research associate with Solvay.

The fact that these materials achieve such high stiffnesses without reinforcements is a big deal. Unfilled plastics typically weigh less than comparable reinforced plastics. One Primospire grade, for example, has a specific gravity of 1.19, versus 1.3 and up for typical reinforced engineering thermoplastic. Unfilled Primospire also has isotropic mechanical properties. Engineers no longer have to worry about the orientation of glass- or carbon-fibers within the plastic matrix--and the resulting directional differences in strength.

What is It?

Primospire materials are based on a benzoyl-substituted para-phenylene chemical structure. “The vast majority of high performance engineering polymers have phenylene units as their key building block,” says El-Hibri. But other polymers also contain molecular connecting groups – esters, for example – that act as weak spots in the polymer backbone. A true polyphenylene, Primospire foregoes these weakening groups and has only phenylene-to-phenylene bonds.

These materials fall into a broader category known as “rigid rod” polymers. Their chemistries differ, but their mechanical properties come from relatively long, stiff molecules that in essence allow the polymer to reinforce itself. In fact, Solvay uses the term “self reinforcing polymer” rather than “rigid rod.”

Whatever you call them, this class of polymer isn’t new. The military, chemical companies and universities have been working with various rigid-rod polymers since the early 1960s. “All the efforts basically failed because there was no way to make these materials melt processible,” says Nick Malkovich, Solvay’s global manager for defense applications. Without materials that could be molded or extruded, only a few niche applications emerged, and these material have stayed mostly in the lab.

An important breakthrough came when scientists at Mississippi Polymer Technologies (MPT) figured out a way to modify polyphenylene, making it melt-processible through extrusion. The company commercialized the first of these “Parmax” materials in 2004. Solvay later bought MPT and changed the product name to Primospire. This month, Solvay brought out a previously unavailable injection molding grade called Primospire PR-250.

Where To Use It

The combination of high strength and light weight makes the materials not just a good metal-replacement candidate but also a replacement for some reinforced plastics. And that’s exactly what been happening, according to Lorenzo DiSano, a product manager at Ensinger, a leading global supplier of stock plastic shapes. Ensinger has been using Primospire materials for its TecaMax line of machinable stock plastic shapes. DiSano says the materials have started to turn up in four different markets.

“The first one has been defense,” he says. “The materials have strength-to-weight attributes that make them attractive for components that have to fly.” This long list includes everything from electronics housings to fasteners that go on military planes or helicopters. DiSano and Malkovich also hint that the material has drawn interest for use in aerodynamic structures for missiles and as a replacement for metal in ammunition casings.

Civilian aviation uses may follow. El-Hibri notes that the material has good flame resistance properties. “We were awestruck by its oxygen index,” he says, noting that it needs an atmosphere of at least 55 percent oxygen to burn whereas most high performance plastics require between 30 and 40 percent. Solvay has also subjected the Primospire to the OSU heat release test, and El-Hibri reports that unmodified Primospire PR-250, the injection molding grade, has a heat release of 28 KW/m2without the use of flame retardants, compared to heat release rates in the 30’s and 40’s for heavier plastic compounds that do contain flame retardants.

Medical devices are another emerging application area. The material has already been used for fixtures used in orthopedic surgery. DiSano says these fixtures have mostly been made from stainless steel. “The advantage of the Primospire is that it allows x-rays to pass through, rather than blocking them as steel does,” he says. Some users had tried to use carbon-fiber-reinforced plastics for this application, “but machining left carbon fibers exposed on the surface,” he says.

Many more medical applications will come in the future, DiSano predicts. He notes that the Primospire materials can tolerate ETO and steam sterilization. They’re also USP Class VI compliant.

Primospire materials have also shown unexpectedly broad chemical resistance and good wear properties (see chart), which makes them a good fit for parts used in chemical process industries and for a variety of semiconductor wafer handling components, DiSano says.

Room for Improvement

For all its beneficial properties, there’s still plenty of room to improve Primospire. Solvay is looking at it as a component in blends or alloys with its other high-performance materials, El-Hibri reports. One area that could stand some improvement is thermal performance. Primospire materials have a glass-transition temperature of 316 F for an extrusion grade and 334 F for a molding grade. “That’s not extremely high,” says El-Hibri, “though the materials do have an HDT that’s very close to their glass transition temperature,” meaning that they retain their stiffness as the temperature rises.

Ensinger is also looking for ways to build improve the properties of the base material. The company has efforts underway to reinforce Primospire with carbon nanotubes - “tofurther improve modulus and creep performance,” says DiSano. The company has also added a proprietary functionalized nanoceramic, called EWS, to the Primospire as a way to improve abrasion resistance and hardness. This latter compound will be aimed at applications such as CMP rings in semiconductor manufacturing, DiSano reports.

Solvay’s Primospire materials are currently available in two grades - PR-120 for extrusion and PR-250 for injection molding. Get more information and download data sheets on each on Solvay's Primospire materials. Ensinger sells stock shapes based on Primospire under the TecaMax brand name.

Primospire materials, shown here as plastic stock shapes for machining, are the stiffest unreinforced thermoplastics that you can buy. The material has just been made available as an injection molding grade too.

Next to other high-end engineering thermoplastics, Primospire exhibits superior wear characteristics. (Source: Ensinger)